tools: zlib: do not hardcode the install prefix in zlib.pc

Our pkg-config wrapper relies on the ability to redefine the $prefix and
$exec_prefix variables in order to construct proper search paths relative
to the build environment.

Patch the .pc file template to construct libdir, sharedlibdir and includedir
relative to the ${prefix} variable so that it can be overridden as needed.

This also fixes the libxml2/host build issue raised at
https://github.com/openwrt/packages/issues/6073 - it was caused by libxml2's
configure picking up a wrong host search path through zlib.pc, letting it
include the wrong endian.h, causing spurious member redeclaration errors in
system headers.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

tools/zlib/patches/900-overridable-pc-exec-prefix.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=${prefix}/lib
++sharedlibdir=${prefix}/lib
++includedir=${prefix}/include
+ 
+ Name: zlib
+ Description: zlib compression library