Add a keep_system_flag

[scarlehoff]

This PR adds keep_system flag for cflags and libs to be consistent with pkgconfig:

~$ pkgconf --libs glib-2.0
-lglib-2.0
~$ pkgconf --libs glib-2.0 --keep-system-libs
-L/usr/lib -lglib-2.0

In python:

>>> import pkgconfig
>>> pkgconfig.cflags('glib-2.0')
'-I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include'
>>> pkgconfig.cflags('glib-2.0', keep_system=True)
'-I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include'
>>> pkgconfig.libs('glib-2.0')
'-lglib-2.0'
>>> pkgconfig.libs('glib-2.0', keep_system=True)
'-L/usr/lib -lglib-2.0'