Error detecting -fno-rtti -fno-exceptions availability under GCC 4

[Luca Fascione]

Hello,
here is my situation:

This script

// configure.in
AC_INIT(README)
AC_CANONICAL_SYSTEM
AM_CONFIG_HEADER(config.h)

AC_CHECK_TOOL(CC, gcc, gcc)

AM_PROG_LIBTOOL

Will go and run a macro called AC_LIBTOOL_PROG_COMPILER_NO_RTTI
which tests the availability of the compiler options -fno-rtti 
-fno-exceptions by running

gcc -c -g -O2  -fno-rtti -fno-exceptions  conftest.c >&5

This, unfortunately, breaks under gcc 4, because the .c extension puts 
the compiler in C mode (as opposed to C++), hence having it refuse the 
C++ specific options as above.

A possible fix would be to run

gcc -c -g -O2  -fno-rtti -fno-exceptions -x c++ conftest.c >&5

(for example having the macro expand to something on the lines of

AC_LIBTOOL_COMPILER_OPTION([if $compiler supports -fno-rtti 
-fno-exceptions],
   lt_cv_prog_compiler_rtti_exceptions,
   [-fno-rtti -fno-exceptions -x c++], [],
   [_LT_AC_TAGVAR(lt_prog_compiler_no_builtin_flag, 
$1)="$_LT_AC_TAGVAR(lt_prog_compiler_no_builtin_flag, $1) -fno-rtti 
-fno-exceptions"])

note the added -x c++ only to the actualy compiler invocation).

This behaviour works fine on gcc 2.96, 3.3.2, 3.4.0, 4.0.1 (which are 
the ones I could test)

Thank you
Luca

--
Luca Fascione
Pipeline Engineer - Weta Digital

+644 380 9815 (x4904) / +64 21 0764 862