Summary:
Ignore /usr as prefix or kdedir when prepending environment variables to not
overrule user setting of PKG_CONFIG_PATH.

For me it was enough to fix my build with prepending CMAKE_PREFIX_PATH
conditionally, but in theory the other environment variables should also only
be prepended if it is not /usr to prepend.

Test Plan:
Compiled XCB 1.13 to /opt/xcb and have XCB 1.11 in /usr/.... Set
PKG_CONFIG_PATH to /opt/xcb/lib/pkgconfig.

Without this patch CMake, called by kdesrc-build, finds XCB 1.11 in
/usr/lib/x86_64-linux-gnu according to its log, but sets as version number
1.13 for some reason.

On make this fails. It still uses the lib from /opt/xcb> but does not include
the correct headers in opt/xcb/include.

With the patch CMake directly finds the lib in /opt/xcb and build succeeds.

Reviewers: bshah, mpyne

Reviewed By: mpyne

Subscribers:

Tags:

Differential Revision: https://phabricator.kde.org/D12739