build: Don't error out on missing cairo-xlib.pc

When building on OSX, we would be using Cairo without XLib support, so
instead of erroring out if pkg-config can't find cairo-xlib, print a
warning. The X-based code will not be used anyway if Cairo hasn't
detected XLib.
build: Use -R instead of -rpath

When linking with libtool, you are supposed to use -R, not -rpath. This
caused a build failure on OSX due to two -rpath options being given in
one invocation of ld, which libtool fixes when you use -R.
build: Use m4 directory for macros

Previously, ACLOCAL_AMFLAGS was set to ${ACLOCAL_FLAGS}. This caused
problems when running libtoolize --force --copy as part of autoreconf.
On JHBuild when ACLOCAL_FLAGS was set to the build tree's share/aclocal
directory, libtoolize would actually remove the libtool macros from
there.