Do not generate C resources for all possible toolchains

The resources data is generated for both GCC and MSVC toolchains, even
though we know beforehand which toolchain we're going to compile it for.
By dropping the data duplication we make the generated resources file
faster to compile, especially when dealing with large embedded data,
instead of relying on the C pre-processor to walk the whole file and
discard the branch we're not using.

docs/reference/gio/glib-compile-resources.xml

@@ -200,6 +200,16 @@ be used with <literal>make</literal>.
 </para></listitem>
 </varlistentry>
 
+<varlistentry>
+<term><option>--compiler=<replaceable>NAME</replaceable></option></term>
+<listitem><para>
+Generate code that is going to target the given compiler <replaceable>NAME</replaceable>.
+The current two compiler modes are "gcc", for all GCC-compatible toolchains; and "msvc",
+for the Microsoft Visual C Compiler. If this option isn't set, then the default will be
+taken from the <envar>CC</envar> environment variable.
+</para></listitem>
+</varlistentry>
+
 </variablelist>
 </refsect1>
 

gio/glib-compile-resources.c

@@ -710,6 +710,86 @@ escape_makefile_string (const char *string)
   return g_string_free (str, FALSE);
 }
 
+typedef enum {
+  COMPILER_GCC,
+  COMPILER_CLANG,
+  COMPILER_MSVC,
+  COMPILER_UNKNOWN
+} CompilerType;
+
+/* Get the compiler id from the platform, environment, or command line
+ *
+ * Keep compiler IDs consistent with https://mesonbuild.com/Reference-tables.html#compiler-ids
+ * for simplicity
+ */
+static CompilerType
+get_compiler_id (const char *compiler)
+{
+  char *base, *ext_p;
+  CompilerType compiler_type;
+
+  if (compiler == NULL)
+    {
+#ifdef G_OS_UNIX
+      const char *compiler_env = g_getenv ("CC");
+
+# ifdef __APPLE__
+      if (compiler_env == NULL || *compiler_env == '\0')
+        compiler = "clang";
+      else
+        compiler = compiler_env;
+# elif __linux__
+      if (compiler_env == NULL || *compiler_env == '\0')
+        compiler = "gcc";
+      else
+        compiler = compiler_env;
+# else
+      if (compiler_env == NULL || *compiler_env == '\0')
+        compiler = "unknown";
+      else
+        compiler = compiler_env;
+# endif
+#endif
+
+#ifdef G_OS_WIN32
+      if (g_getenv ("MSYSTEM") != NULL)
+        {
+          const char *compiler_env = g_getenv ("CC");
+
+          if (compiler_env == NULL || *compiler_env == '\0')
+            compiler = "gcc";
+          else
+            compiler = compiler_env;
+        }
+      else
+        compiler = "msvc";
+#endif
+    }
+
+  base = g_path_get_basename (compiler);
+  ext_p = strrchr (base, '.');
+  if (ext_p != NULL)
+    {
+      gsize offset = ext_p - base;
+      base[offset] = '\0';
+    }
+
+  compiler = base;
+
+  if (g_strcmp0 (compiler, "gcc") == 0)
+    compiler_type = COMPILER_GCC;
+  else if (g_strcmp0 (compiler, "clang") == 0)
+    compiler_type = COMPILER_CLANG;
+  else if (g_strcmp0 (compiler, "msvc") == 0)
+    compiler_type = COMPILER_MSVC;
+  else
+    compiler_type = COMPILER_UNKNOWN;
+
+  g_free (base);
+
+  return compiler_type;
+}
+
 int
 main (int argc, char **argv)
 {
@@ -732,6 +812,8 @@ main (int argc, char **argv)
   char *c_name = NULL;
   char *c_name_no_underscores;
   const char *linkage = "extern";
+  const char *compiler = NULL;
+  CompilerType compiler_type = COMPILER_GCC;
   GOptionContext *context;
   GOptionEntry entries[] = {
     { "version", 0, 0, G_OPTION_ARG_NONE, &show_version_and_exit, N_("Show program version and exit"), NULL },
@@ -747,6 +829,7 @@ main (int argc, char **argv)
     { "internal", 0, 0, G_OPTION_ARG_NONE, &internal, N_("Don’t export functions; declare them G_GNUC_INTERNAL"), NULL },
     { "external-data", 0, 0, G_OPTION_ARG_NONE, &external_data, N_("Don’t embed resource data in the C file; assume it's linked externally instead"), NULL },
     { "c-name", 0, 0, G_OPTION_ARG_STRING, &c_name, N_("C identifier name used for the generated source code"), NULL },
+    { "compiler", 'C', 0, G_OPTION_ARG_STRING, &compiler, N_("The target C compiler (default: the CC environment variable)"), NULL },
     G_OPTION_ENTRY_NULL
   };
 
@@ -802,6 +885,8 @@ main (int argc, char **argv)
   if (internal)
     linkage = "G_GNUC_INTERNAL";
 
+  compiler_type = get_compiler_id (compiler);
+
   srcfile = argv[1];
 
   xmllint = g_strdup (g_getenv ("XMLLINT"));
@@ -1104,9 +1189,10 @@ main (int argc, char **argv)
                      data_size, c_name);
         }
       else
+        {
+          if (compiler_type == COMPILER_MSVC || compiler_type == COMPILER_UNKNOWN)
             {
               /* For Visual Studio builds: Avoid surpassing the 65535-character limit for a string, GitLab issue #1580 */
-          g_fprintf (file, "#ifdef _MSC_VER\n");
               g_fprintf (file,
                          "static const SECTION union { const guint8 data[%"G_GSIZE_FORMAT"]; const double alignment; void * const ptr;}  %s_resource_data = { {\n",
                          data_size + 1 /* nul terminator */, c_name);
@@ -1123,9 +1209,9 @@ main (int argc, char **argv)
                 }
 
               g_fprintf (file, "} };\n");
-
+            }
-          /* For other compilers, use the long string approach */
+          else
-          g_fprintf (file, "#else /* _MSC_VER */\n");
+            {
               g_fprintf (file,
                          "static const SECTION union { const guint8 data[%"G_GSIZE_FORMAT"]; const double alignment; void * const ptr;}  %s_resource_data = {\n  \"",
                          data_size + 1 /* nul terminator */, c_name);
@@ -1138,7 +1224,7 @@ main (int argc, char **argv)
                 }
 
               g_fprintf (file, "\" };\n");
-          g_fprintf (file, "#endif /* !_MSC_VER */\n");
+            }
         }
 
       g_fprintf (file,