Instead of hard-coding lib64 and other include locations, just prepend
the DRM_DIR to the beginning of the CMake prefix path. Then let
pkgconfig find the package, the same way that it would if DRM_DIR wasn't
set. DRM_DIR takes precedence, but the default paths will be used if
DRM_DIR isn't set, or doesn't point to where libdrm is housed
Note that /lib and /lib/$ARCH aren't required for DRM_DIR, just the
path to the root folder for the package (e.g. /opt/amdgpu instead of
/opt/amdgpu/lib or /opt/amdgpu/lib64 or /opt/amdgpu/lib/x86_64-linux-gnu
etc)
Change-Id: I56767db28476d14e3fa77be1089c3904e2a32450
Kent Russell
d0c2770cde