girepository: Always use currently built GLib to run g-ir-scanner

If we use the system libraries we can't really debug it, so ensure we're
using the currently built libraries for it too.

We're doing it globally to ensure that we're testing the result that
this version of the libraries are building

girepository/introspection/meson.build

@@ -19,6 +19,12 @@ gi_gen_shared_dependencies = [
 
 gi_gen_env_variables = environment()
 
+# Use currently built libraries to run g-ir-scanner and the various tools
+# this may not happen if we don't set the library paths.
+gi_gen_env_variables.prepend(glib_exec_var_library_path,
+  fs.parent(libglib.full_path()), fs.parent(libgobject.full_path()),
+  fs.parent(libgmodule.full_path()), fs.parent(libgio.full_path()))
+
 if 'address' in glib_sanitizers
   gi_gen_env_variables.append(
     'ASAN_OPTIONS', glib_exec_asan_option_ignore_preload, separator: ',')

meson.build

@@ -2673,9 +2673,11 @@ glib_exec_preloaded_libs = []
 glib_exec_preloaded_env = {}
 
 if host_system in ['ios', 'darwin']
+  glib_exec_var_library_path = 'DYLD_LIBRARY_PATH'
   glib_exec_var_preload = 'DYLD_INSERT_LIBRARIES'
   glib_exec_var_preload_separator = ':'
 else
+  glib_exec_var_library_path = 'LD_LIBRARY_PATH'
   glib_exec_var_preload = 'LD_PRELOAD'
   glib_exec_var_preload_separator = ' '
 endif