AC_CHECK_HEADER question

[Mattias Brändström]

Hello!

I am getting some strange errors when using AC_CHECK_HEADER. I want to
check for a specific header file and I do the following in my
configure.ac:

AC_LANG_PUSH([C++])
AC_CHECK_HEADER([SDL_ttf.h], [],
                AC_MSG_ERROR([SDL_ttf.h header file required]))
AC_LANG_POP

The file SDL_ttf.h resides in /usr/local/include/SDL. When I look in
config.log I see the following:

configure:9280: checking SDL_ttf.h usability
configure:9293: g++ -c -ggdb -O0 -Wall -I/usr/include/SDL -D_REENTRANT
conftest.cc >&5

This works fine. After the ebove test ./configure does this:

configure:9316: checking SDL_ttf.h presence
configure:9327: g++ -E  conftest.cc
configure:9343:21: SDL_ttf.h: No such file or directory

Which fails since it doesn't seem to use my CXXFLAGS variable at all. I
also get this message from ./configure:

configure: WARNING: SDL_ttf.h: accepted by the compiler, rejected by the
preprocessor!
configure: WARNING: SDL_ttf.h: proceeding with the preprocessor's result
configure: WARNING:     ## ------------------------------------ ##
configure: WARNING:     ## Report this to address@hidden ##
configure: WARNING:     ## ------------------------------------ ##

Is this a bug? What is the difference between checking for usability and
checking for presence? Shouldn't autoconf use my CXXFLAGS when checking
for a header files presence?

Regards,
Mattias