You can do something like this:

PKG_CHECK_MODULES(CAIRO, cairo >= 1.xx, cairohead=yes, cairohead=no)

That'll check the version and then you can set a variable that'll get
passed to the codebase like:

if test "x$cairohead" = "xyes"; then
       AC_DEFINE(ENABLE_SVG_FONTS, 1, [SVG Fonts should be used])
fi

And that'll be put in config.h so that you can test for it in your code.

               --Ted