improve package detection

ax_init_standard_project.m4

@@ -412,13 +412,13 @@ AC_DEFUN([AX_PKG_REQUIRE], [
     old_CPPFLAGS=${CPPFLAGS}
     CPPFLAGS=" ${$1_CFLAGS} ${CPPFLAGS}"
     AC_CHECK_HEADER([$3], [
-      $1_found=yes
+      $1_found=${tmp_package}
     ], [
       for x in ${$1_CFLAGS}; do
         AC_MSG_NOTICE([search for $3 in ${x[#]-I}])
         for f in $(find ${x[#]-I} -name "$3"); do
           if test -f "$f"; then
-            $1_found=yes
+            $1_found=${tmp_package}
             $1_CFLAGS+=" -I${f%/*}"
             AC_MSG_NOTICE([added path ${f%/*}])
             break;
@@ -434,7 +434,7 @@ AC_DEFUN([AX_PKG_REQUIRE], [
           AC_MSG_NOTICE([search for $3 in $x])
           for f in $(find ${x} -name "$3"); do
             if test -f "$f"; then
-              $1_found=yes
+              $1_found=${tmp_package}
               $1_CFLAGS+=" -I${f%/*}"
               AC_MSG_NOTICE([added path ${f%/*}])
               break;