Encode Epoxy capabilities into the pkg-config file

Since Epoxy can be built with different platform-specific API, having a
way for dependent projects to discover those capabilities without
necessarily crashing the minute they attempt to use them is a good
feature to have.

We strongly direct library and application developers to use pkg-config
in order to use Epoxy, so it makes sense to add variables to the
epoxy.pc file that can be easily extracted at configuration time.

configure.ac

@@ -205,6 +205,15 @@ AM_CONDITIONAL(HAVE_X11, test x$x11 = xyes)
 
 PKG_CHECK_MODULES(GL, [gl], [gl=yes], [gl=no])
 
+# Variables for the pkg-config file; AC_SUBST does not do `test` substitutions,
+# so we need to specify the boolean values here
+AS_IF([test x$build_glx = xyes], [epoxy_has_glx=1], [epoxy_has_glx=0])
+AS_IF([test x$build_egl = xyes], [epoxy_has_egl=1], [epoxy_has_egl=0])
+AS_IF([test x$build_wgl = xyes], [epoxy_has_wgl=1], [epoxy_has_wgl=0])
+AC_SUBST(epoxy_has_glx)
+AC_SUBST(epoxy_has_egl)
+AC_SUBST(epoxy_has_wgl)
+
 AC_CONFIG_FILES([
                 epoxy.pc
                 Makefile

epoxy.pc.in

@@ -3,6 +3,10 @@ exec_prefix=@exec_prefix@
 libdir=@libdir@
 includedir=@includedir@
 
+epoxy_has_glx=@epoxy_has_glx@
+epoxy_has_egl=@epoxy_has_egl@
+epoxy_has_wgl=@epoxy_has_wgl@
+
 Name: epoxy
 Description: epoxy GL dispatch Library
 Version: @PACKAGE_VERSION@

meson.build

@@ -160,6 +160,9 @@ pkgconf.set('prefix', epoxy_prefix)
 pkgconf.set('exec_prefix', epoxy_prefix)
 pkgconf.set('libdir', epoxy_libdir)
 pkgconf.set('includedir', epoxy_includedir)
+pkgconf.set10('epoxy_has_glx', build_glx)
+pkgconf.set10('epoxy_has_egl', build_egl)
+pkgconf.set10('epoxy_has_wgl', build_wgl)
 pkgconf.set('PACKAGE_VERSION', meson.project_version())
 if dl_dep.found()
   pkgconf.set('DLOPEN_LIBS', '-ldl')