Use variable library name in pkg-config output.

Use variable library name in pkg-config output.

Previously the pkg-config output files would contain e.g. `-lGL'
and `-lGLU', even if the user modified their configuration to
build libraries with different names.  This modifies the
pkg-config inputs, and corresponding makery, so that modifying the
output library name will cause the appropriate updated name to
appear in the pkg-config `-l' option.

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>