pango: set config arg to create .pc files to support explicit linking

[marino]

Summary

For some reason, FreeBSD 10+ does not require explicit linking
to libm and libz. DragonFly requires explicit linking for *all* libraries
and so there are numerous ports that pass on FreeBSD but fail on DragonFly.
I think I've traced these failures to pango and gtk20 -- their pkgconf
files were not listing all the libraries needed to link. The pango fix
is simple: just add a configure argument.

[marino]

committed