libgd: Fix pkgconfig file for cross compilation

This allows packages that use pkgconfig to find libgd properly.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

libs/libgd/patches/210-generate-install-gdlib.pc-for-cmake-builds-too-164.patch

@@ -14,9 +14,11 @@ new file mode 100644
 index 00000000..fb828882
 --- /dev/null
 +++ b/config/gdlib.pc.cmake
-@@ -0,0 +1,9 @@
-+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
-+includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
+@@ -0,0 +1,11 @@
++prefix=@CMAKE_INSTALL_PREFIX@
++exec_prefix=@CMAKE_INSTALL_PREFIX@
++libdir=${exec_prefix}/@CMAKE_INSTALL_LIBDIR@
++includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
 +
 +Name: gd
 +Description: GD graphics library