autotools: use correct gl.pc LIBS when using glvnd

This is more of a hack, since glvnd itself should be providing the file.
Until that happens, ensure the libs is correctly set to -lGL

CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 315c46cfdc)

configure.ac

@@ -1541,6 +1541,15 @@ fi
 AC_SUBST([GL_LIB])
 AC_SUBST([OSMESA_LIB])
 
+dnl HACK when building glx + glvnd we ship gl.pc, despite that glvnd should do it
+dnl Thus we need to use GL as a DSO name.
+if test "x$enable_libglvnd" = xyes -a "x$enable_glx" != xno; then
+  GL_PKGCONF_LIB="GL"
+else
+  GL_PKGCONF_LIB="$GL_LIB"
+fi
+AC_SUBST([GL_PKGCONF_LIB])
+
 # Check for libdrm
 PKG_CHECK_MODULES([LIBDRM], [libdrm >= $LIBDRM_REQUIRED],
                   [have_libdrm=yes], [have_libdrm=no])

src/mesa/gl.pc.in

@@ -7,7 +7,7 @@ Name: gl
 Description: Mesa OpenGL library
 Requires.private: @GL_PC_REQ_PRIV@
 Version: @PACKAGE_VERSION@
-Libs: -L${libdir} -l@GL_LIB@
+Libs: -L${libdir} -l@GL_PKGCONF_LIB@
 Libs.private: @GL_PC_LIB_PRIV@
 Cflags: -I${includedir} @GL_PC_CFLAGS@
 glx_tls: @GLX_TLS@