PKG_CHECK_MODULES prefix: cairo => CAIRO

Use upper case for the prefix to PKG_CHECK_MODULES, as this seems to be
the standard.
Pass $(CAIRO_CFLAGS) to compiler

Amongst other things, $(CAIRO_CFLAGS) specifies the path to cairo's
header files.
Fix cairo.h include

The include directive should not include the cairo/ subdirectory, but
only the name of the header file itself.  pkg-config(1) provides the
path to find it.
Add more detail to usage message
Add option to display usage
Add option to display version string