- Fix detection of compiler flags with Clang; by default Clang issues a warning
- Fix separate build tree issue

Index: configure.ac
--- configure.ac.orig
+++ configure.ac
@@ -156,12 +156,12 @@ if test -n "${MAINTAINER_MODE_FALSE}"; then
 fi
 
 all_desired_work=false
-AS_COMPILER_FLAG([$DESIRED_FLAGS], [all_desired_work=true])
+AS_COMPILER_FLAG([-Werror $DESIRED_FLAGS], [all_desired_work=true])
 if $all_desired_work ; then
     CFLAGS="$CFLAGS $DESIRED_FLAGS"
 else
 for flag in $DESIRED_FLAGS; do
-  AS_COMPILER_FLAG([$flag], [CFLAGS="$CFLAGS $flag"])
+  AS_COMPILER_FLAG([-Werror $flag], [CFLAGS="$CFLAGS $flag"])
 done
 fi
 
@@ -203,7 +203,7 @@ AH_TEMPLATE([HAVE_GPERF],
 tmp_gperf=false
 if test "x$GPERF" != "x" -a "x$GPERF" != "xno" ; then
   AS_ECHO_N(["checking whether gperf is recent enough... "])
-  if "$GPERF" < src/conf-tokens.gperf >/dev/null 2>&1 ; then
+  if "$GPERF" < $srcdir/src/conf-tokens.gperf >/dev/null 2>&1 ; then
     AS_ECHO("yes")
     AC_DEFINE(HAVE_GPERF)
     tmp_gperf=true
