libbz2: add pkg-config file (bzip2.pc)

Add generation of a bzip2.pc file for use with pkg-config and create
a basic template as bzip2.pc.in.

This allows other software to easily locate and link against libbz2
using standard pkg-config mechanisms instead of manual compiler and
linker flags.

The version number is extracted automatically from bzlib.h to keep
it consistent with the library sources.

Tested: pkg-config --exists --print-errors "bzip2 >= 1.0.9" (fail, expected)
pkg-config --exists --print-errors "bzip2 >= 1.0" (succeeded)
pkg-config --libs bzip2 (-lbz2)
pkg-config --cflags bzip2 ()

MFC after: 3 days