Fix build on systems with no sfcgal installed

See https://travis-ci.org/postgis/postgis/builds/87413334 Ref #3348 git-svn-id: http://svn.osgeo.org/postgis/trunk@14329 b70326c6-7e19-0410-871a-916f4a2858ee

Author: Sandro Santilli

configure.ac

@@ -664,12 +664,12 @@ dnl ===========================================================================
 AC_ARG_WITH([sfcgal], 
 [AS_HELP_STRING([--with-sfcgal=PATH], [Add SFCGAL support. ARG allows to specify an alternate PATH to sfcgal-config])], 
 [SFCGAL_CONFIG="$withval"], 
-[with_sfcgal=yes])
+[with_sfcgal=auto])
 
 HAVE_SFCGAL="no"
 
 if test "x$with_sfcgal" != "xno"; then
- if test "x$with_sfcgal" = "xyes"; then
+ if test "x$with_sfcgal" = "xyes" -o "x$with_sfcgal" = "xauto"; then
  AC_PATH_PROG([SFCGAL_CONFIG], [sfcgal-config], [])
  fi
 if test -x "$SFCGAL_CONFIG"; then
@@ -695,7 +695,7 @@ if test "x$with_sfcgal" != "xno"; then
 fi
 
 else
-if test "x$with_sfcgal" != "x"; then
+if test "x$with_sfcgal" != "xauto"; then
 AC_MSG_ERROR([sfcgal-config cannot be found. Please install sfcgal])
 fi
 fi