zlib: Use relative paths in pkg-config metadata file

The buildroot pkg-config (in staging_dir/host/bin) overrides the prefix
and exec_prefix variables in *.pc files, to supply the correct
(buildroot) paths for callers. If other variables are not defined
relative to prefix and exec_prefix, then the returned values will be
incorrect.

The default zlib.pc file generated by cmake contains absolute paths.
This patches the file to use relative paths (relative to ${prefix} and
${exec_prefix}).

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

package/libs/zlib/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=zlib
 PKG_VERSION:=1.2.11
-PKG_RELEASE:=2
+PKG_RELEASE:=3
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=@SF/libpng http://www.zlib.net

package/libs/zlib/patches/004-relative-pkg-config-paths.patch

@@ -0,0 +1,14 @@
+--- a/zlib.pc.cmakein
++++ b/zlib.pc.cmakein
+@@ -1,8 +1,8 @@
+ prefix=@CMAKE_INSTALL_PREFIX@
+ exec_prefix=@CMAKE_INSTALL_PREFIX@
+-libdir=@INSTALL_LIB_DIR@
+-sharedlibdir=@INSTALL_LIB_DIR@
+-includedir=@INSTALL_INC_DIR@
++libdir=${exec_prefix}/lib
++sharedlibdir=${exec_prefix}/lib
++includedir=${prefix}/include
+ 
+ Name: zlib
+ Description: zlib compression library