pkg-config is unusable for local builds

We use a pkg-config wrapper and pkg-config's sysroot mode to relocate the .pc files that come with Ubuntu.

This causes a prefix=/usr/lib to automatically be converted to prefix=$SYSROOT/usr/lib where sysroot is set in the wrapper script and passed to pkg-config itself.

This falls down if you want to have your own local build tree with local .pc files. Taking the binary test scripts as an example:

michaelh@crucis:~/linaro/binary/linaro/contrib/linaro/tests$ arm-linux-gnueabi-pkg-config --cflags libffi
-I/home/michaelh/linaro/binary/check/gcc-linaro-arm-linux-gnueabi-2011.12-20111219+bzr2299~linux/bin/../arm-linux-gnueabi/libc/home/michaelh/linaro/binary/linaro/contrib/linaro/tests/install/lib/libffi-3.0.10/include

Change pkg-config to only insert the sysroot if the prefix starts with /usr or similar?