glx: Prefer library path given by pkgconfig over the system

glx: Prefer library path given by pkgconfig over the system

Recent change to use drmGetDevices2() made me realize that
build configured using

PKG_CONFIG_PATH=my_drm_lib_path/pkgconfig ./autogen.sh

considers the libdrm path gotten from pkgconfig only during
make. When invoking "make install" the relink command puts
system library ahead of the path gotten from pkgconfig
(and starts to fail as system libdrm isn't new enough).

This change forces the relink command to respect pkgconfig
settings.

It looks to me that in

https://bugs.freedesktop.org/show_bug.cgi?id=100259

with Emil et al considering it a libtool bug.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
[Emil Velikov: add inline comment]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>