Added pkg-config support

This patch adds pkg-config support. This has been tested on OS X and FreeBSD using the expat parser. No tests with the libxml parser have been made.
`pkg-config --libs libstrophe` should print all dependencies. Better use `Requires:` if *.pc file exists in a system. Something like this:
```

PKG_CHECK_MODULES([libxml2], [libxml-2.0 >= 2.7],
        [with_libxml2=yes; PC_REQUIRES="$PC_REQUIRES libxml-2.0 >= 2.7"],
        [PC_LIBS="$PC_LIBS -lxml2"])

libstrophe.pc:
  Requires: @PC_REQUIRES@
  Libs: -L${libdir} -lstrophe @PC_LIBS@
```