pkg-config files include cflags for static builds
On windows, if shared libraries are disabled, then pkg-config --cflags
with files libxslt.pc and libexslt.pc should report the -DLIBXSLT_STATIC
and -DLIBEXSLT_STATIC
, respectively.
Related to #73 (closed) and f9a0817b which fixed this for the xslt-config
script.