Respect PKG_CONFIG variable if set in the environment. This is required for the new cross-compilation support using pkg-config wrappers instead of changing the pkg-config files themselves. BUG=none TEST=emerge-arm-generic with and without pkg-config wrappers Change-Id: I46d17ccea3d367796ada548822eec9dd8dc104fe Reviewed-on: http://gerrit.chromium.org/gerrit/1438 Reviewed-by: Daniel Erat <[email protected]> Tested-by: Diego Elio Pettenò <[email protected]>