This enables various compiler warnings project wide and disables the triggered
ones for each library/executable. This should give us roughly the same behaviour
as with autotools.
Tested with gcc8 and clang7.
This adds the following macros and functions: GI_MAJOR_VERSION, GI_MICRO_VERSION,
GI_MINOR_VERSION, GI_CHECK_VERSION, gi_get_major_version,gi_get_micro_version,
gi_get_minor_version.
Since we share a prefix with glib we have to namespace these by using the gi_
prefix. g_gi would also work but we already export symbols with gi_ like
gi_cclosure_marshal_generic(), gi_type_tag_get_ffi_type() and
gi_type_info_extract_ffi_return_value(), so let's not add another naming scheme.
When building with Meson, we cannot set environment variables while
running custom targets and our builddir layout is different from
Autotools anyway.
Now g-ir-scanner and friends can autodetect when they're being run
uninstalled by Meson and will find _giscanner.so and the giscanner
python files in the build directory. This is very similar to what
gdbus-codegen uses in glib/gio.
Same for girepository/gdump.c.
glib_dep is what is actually needed to #include <glib.h>, not
gobject_dep. It works incidentally with system gobject/glib but not
when built via subprojects.
