Add gl and egl private dependencies

If the system we're building Epoxy on has GL and EGL pkg-config modules,
then we should add them to the Requires.private field of the pkg-config
file.

The Requires.private field does not contribute to the linker flags
generated by pkg-config, unless we're doing a static build; it does,
however, contribute to the compiler flags generated by pkg-config, which
means that platforms that specify ad hoc compiler flags for their GL and
EGL implementations via pkg-config will be able to propagate them
through Epoxy.

Closes: #139

configure.ac

@@ -227,6 +227,12 @@ fi
 AM_CONDITIONAL(HAVE_X11, test x$x11 = xyes)
 
 PKG_CHECK_MODULES(GL, [gl], [gl=yes], [gl=no])
+PKG_CHECK_MODULES(EGL, [egl], [egl=yes], [egl=no])
+
+GL_REQS=""
+AS_IF([test x$gl = xyes], [GL_REQS="$GL_REQS gl"])
+AS_IF([test x$build_egl = xyes && test x$egl = xyes], [GL_REQS="$GL_REQS egl"])
+AC_SUBST(GL_REQS)
 
 # Variables for the pkg-config file; AC_SUBST does not do `test` substitutions,
 # so we need to specify the boolean values here

epoxy.pc.in

@@ -13,3 +13,4 @@ Version: @PACKAGE_VERSION@
 Cflags: -I${includedir}
 Libs: -L${libdir} -lepoxy
 Libs.private: @DLOPEN_LIBS@
+Requires.private: @GL_REQS@

src/meson.build

@@ -89,6 +89,17 @@ epoxy_has_glx = build_glx ? '1' : '0'
 epoxy_has_egl = build_egl ? '1' : '0'
 epoxy_has_wgl = build_wgl ? '1' : '0'
 
+# We don't want to add these dependencies to the library, as they are
+# not needed when building Epoxy; we do want to add them to the generated
+# pkg-config file, for consumers of Epoxy
+gl_reqs = []
+if gl_dep.found()
+  gl_reqs += 'gl'
+endif
+if build_egl and egl_dep.found()
+  gl_reqs += 'egl'
+endif
+
 pkg = import('pkgconfig')
 pkg.generate(
   libraries: libepoxy,
@@ -101,4 +112,5 @@ pkg.generate(
     'epoxy_has_wgl=@0@'.format(epoxy_has_wgl),
   ],
   filebase: 'epoxy',
+  requires_private: ' '.join(gl_reqs),
 )