even more improved way to find the necessary include path

ax_init_standard_project.m4

@@ -372,12 +372,13 @@ EOF
 # fails if not found
 AC_DEFUN([AX_PKG_REQUIRE], [
   PKG_PROG_PKG_CONFIG
+  $1_found=0
   PKG_CHECK_MODULES([$1], [$2], [
+    $1_found=1
     PKG_REQUIREMENTS+=" $2"
   ], [
     if test -n "$4"; then
       AC_MSG_WARN([Recommended package $2 for feature $1 not installed, trying $4])
-      $1_found=0
       for pkg in $4; do
         PKG_CHECK_MODULES([$1], [$pkg], [
           PKG_REQUIREMENTS+=" $pkg"
@@ -387,22 +388,18 @@ AC_DEFUN([AX_PKG_REQUIRE], [
           AC_MSG_WARN([Recommended package $pkg for feature $1 not installed])
         ])
       done
-      if test ${$1_found} -eq 0; then
-        AC_MSG_ERROR([Required package $2 or $4 for feature $1 not installed])
-      fi
-    else
-      AC_MSG_ERROR([Required package $2 for feature $1 not installed])
     fi
   ])
   AC_SUBST(AM_CPPFLAGS)
   AC_SUBST(AM_CXXFLAGS)
   AC_SUBST(PKG_REQUIREMENTS)
   if test -n "$3"; then
+    $1_found=0
     old_CPPFLAGS=${CPPFLAGS}
     CPPFLAGS=" ${$1_CFLAGS} ${CPPFLAGS}"
     AC_CHECK_HEADER([$3], [
+      $1_found=1
     ], [
-      $1_found=0
       for x in ${$1_CFLAGS}; do
         AC_MSG_NOTICE([search for $3 in ${x[#]-I}])
         for f in $(find ${x[#]-I} -name "$3"); do
@@ -417,12 +414,12 @@ AC_DEFUN([AX_PKG_REQUIRE], [
           break;
         fi
       done
-      if test ${$1_found} -eq 0; then
-        AC_MSG_ERROR([Required header $3 for feature $1 with ${$1_CFLAGS} not found])
-      fi
     ])
     CPPFLAGS=${old_CPPFLAGS}
   fi
+  if test ${$1_found} -eq 0; then
+    AC_MSG_ERROR([Feature $1 not found, need header $3 in module $2 $4])
+  fi
   AM_CPPFLAGS+=" ${$1_CFLAGS}"
   AM_CXXFLAGS+=" ${$1_CFLAGS}"
   LIBS+=" ${$1_LIBS}"