The autoconf macros AX_COMPILER_FLAGS_{CFLAGS|CXXFLAGS|LDFLAGS} test
for compiler and linker support of various flags, and add the flags to
the generated output.

If the command-line option '--enable-compile-warnings' is specified to
'configure', a number of additional warning options is also added to the
output. This is the default.

The AX_COMPILER_FLAGS_* macros add stricter warnings then before. The
patch disables some of them to make dbus build without errors. A later
patch set should fix the warnings and remove the compiler flags.

This patch integrates all tests for compiler flags into the call to
AX_COMPILER_FLAGS_CFLAGS. All tests for compiler flags are now done
in a single place. The old macros have been removed.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
[smcv: add missing $ to DISABLE_WARNINGS]
[smcv: drop -Wno-discarded-qualifiers]
[smcv: drop non-C++ option -Wpointer-sign in C++ mode]
[smcv: work around an AX_C...