build: Improve tests for frexpl() and ldexpl()

The test programs for those in the Meson build files will not work for
Visual Studio prior to 2013 (whereas the rest of the code does).
Improve the tests for these by:

-Adding a test to see whether we can re-define a prototype for these
 functions, using cc.compiles().  If so, set HAVE_DECL_xxxx to be 0,
 otherwise set HAVE_DECL_xxxx to be 1.

Also, for glib/gnulib/frexpl.c, don't undefine frexpl on Visual Studio,
otherwise we will not be able to compile/link it on Visual Studio
compilers.

glib/gnulib/frexpl.c

@@ -14,6 +14,8 @@ long double rpl_frexpl (long double x, int *expptr)
     return x;
   else if (isinf (x))
     return x;
+#ifndef _MSC_VER
 #undef frexpl
+#endif
   return frexpl (x, expptr);
 }

glib/gnulib/gl_cv_func_frexpl_works/meson.build

@@ -29,7 +29,11 @@ extern
 #ifdef __cplusplus
 "C"
 #endif
+
+#if !defined (_MSC_VER) || defined (TEST_FREXPL_DECL)
 long double frexpl (long double, int *);
+#endif
+
 int main()
 {
   int result = 0;
@@ -128,3 +132,11 @@ else
     gl_cv_func_frexpl_broken_beyond_repair = false
   endif
 endif
+
+frexpl_test_decl = '''
+#define TEST_FREXPL_DECL 1
+''' + frexpl_test
+
+build_result = cc.compiles(frexpl_test_decl,
+    name : 'frexpl prototype can be re-listed')
+gl_cv_func_frexpl_decl = build_result

glib/gnulib/gl_cv_func_ldexpl_works/meson.build

@@ -9,7 +9,11 @@ extern
 #ifdef __cplusplus
 "C"
 #endif
+
+#if !defined (_MSC_VER) || defined (TEST_LDEXPL_DECL)
 long double ldexpl (long double, int);
+#endif
+
 int main()
 {
   int result = 0;
@@ -43,3 +47,11 @@ else
     gl_cv_func_ldexpl_works = true
   endif
 endif
+
+ldexpl_test_decl = '''
+#define TEST_LDEXPL_DECL 1
+''' + ldexpl_test
+
+build_result = cc.compiles(ldexpl_test_decl,
+    name : 'ldexpl prototype can be re-listed')
+gl_cv_func_ldexpl_decl = build_result

glib/gnulib/meson.build

@@ -282,6 +282,7 @@ endif
 
 math_h_config.set ('REPLACE_FREXP', gl_cv_func_frexp_works ? 0 : 1)
 math_h_config.set ('REPLACE_FREXPL', gl_cv_func_frexpl_works ? 0 : 1)
+math_h_config.set ('HAVE_DECL_FREXPL', gl_cv_func_frexpl_decl ? 0 : 1)
 
 math_h_config.set ('REPLACE_ITOLD', 0)
 math_h_config.set ('REPLACE_HUGE_VAL', 0)
@@ -293,6 +294,7 @@ else
   gl_cv_func_ldexpl_works = false
 endif
 math_h_config.set ('REPLACE_LDEXPL', gl_cv_func_ldexpl_works ? 0 : 1)
+math_h_config.set ('HAVE_DECL_LDEXPL', gl_cv_func_ldexpl_decl ? 0 : 1)
 
 math_h = configure_file (input: 'gnulib_math.h.in',
                          output: 'gnulib_math.h',