Merge branch 'trac3119'

configure.ac

@@ -755,7 +755,24 @@ CPPFLAGS_SAVED=$CPPFLAGS
 CPPFLAGS="$BOTAN_INCLUDES $CPPFLAGS"
 LIBS_SAVED="$LIBS"
 LIBS="$LIBS $BOTAN_LIBS"
-AC_CHECK_HEADERS([botan/botan.h],,AC_MSG_ERROR([Missing required header files.]))
+
+# ac_header_preproc is an autoconf symbol (undocumented but stable) that
+# is set if the pre-processor phase passes. Thus by adding a custom
+# failure handler we can detect the difference between a header not existing
+# (or not even passing the pre-processor phase) and a header file resulting
+# in compilation failures.
+AC_CHECK_HEADERS([botan/botan.h],,[
+	if test "x$ac_header_preproc" = "xyes"; then
+		AC_MSG_ERROR([
+botan/botan.h was found but is unusable. The most common cause of this problem
+is attempting to use an updated C++ compiler with older C++ libraries, such as
+the version of Botan that comes with your distribution. If you have updated
+your C++ compiler we highly recommend that you use support libraries such as
+Boost and Botan that were compiled with the same compiler version.])
+	else
+		AC_MSG_ERROR([Missing required header files.])
+	fi]
+)
 AC_LINK_IFELSE(
         [AC_LANG_PROGRAM([#include <botan/botan.h>
                           #include <botan/hash.h>