String literals are const in C++. Using this warnings moves us closer to
the stated goal of C++ compatibility, makes the code cleaner and lets
compilers optimize the code better.
Add m4/ax_append_compile_flags.m4 from Autoconf Archives for the
AX_APPEND_COMPILE_FLAGS macro and m4/ax_check_compile_flag.m4 as its
dependency.
Pavel Roskin