Use absolute paths for libraries

To reduce the risk of duplicating the libraries on the link line, as we
have absolute paths in the .la files.

unix/xserver/hw/vnc/Makefile.am

@@ -1,5 +1,5 @@
-TIGERVNC_SRCDIR=${top_srcdir}/../..
-TIGERVNC_BUILDDIR=${TIGERVNC_SRCDIR}
+TIGERVNC_SRCDIR=$(abspath ${top_srcdir}/../..)
+TIGERVNC_BUILDDIR=$(abspath ${TIGERVNC_SRCDIR})
 
 RFB_LIB=$(TIGERVNC_BUILDDIR)/common/rfb/librfb.la
 RDR_LIB=$(TIGERVNC_BUILDDIR)/common/rdr/librdr.la