Add pkg-config file generation support

Fixes #61 

That's generated by autoconf ;-)
On Oct 1, 2014 6:05 PM, "Iñaki Baz Castillo" <notifications@github.com>
wrote:

> In configure:
>
> > +    break 2
> > +  fi
> > +done
> > +  done
> > +IFS=$as_save_IFS
> > +
> > +fi
> > +fi
> > +PKG_CONFIG=$ac_cv_prog_PKG_CONFIG
> > +if test -n "$PKG_CONFIG"; then
> > +  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $PKG_CONFIG" >&5
> > +$as_echo "$PKG_CONFIG" >&6; }
> > +else
> > +  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
> > +$as_echo "no" >&6; }
> > +fi
>
> wow...
>
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> <https://github.com/cisco/libsrtp/pull/70/files#r18288497>.
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