build: Define builddir and abs_srcdir when they are missing.

RHEL 5-era autoconf did not define these, so define them manually
when they are missing.

Define builddir as '.'  The scripts require this.  It won't work
in the srcdir != builddir case, but we don't care about that for
RHEL 5.

This commit also moves the builddir / abs_srcdir variable setting
above the include of subdir-rules.mk, in case that include uses
these variables.

Useful script:

  for f in $(find -name Makefile.am | xargs fgrep '$(abs_srcdir)' -l) ; do
    if ! grep -q '^abs_srcdir' $f; then
      echo missing in $f
    fi
  done

php/Makefile.am

@@ -17,6 +17,9 @@
 
 include $(top_srcdir)/subdir-rules.mk
 
+# Old RHEL 5 autoconf doesn't have abs_srcdir.
+abs_srcdir ?= $(shell cd $(top_srcdir)/php && pwd)
+
 generator_built = \
 	extension/php_guestfs_php.h \
 	extension/guestfs_php.c