Enable maximum compiler warnings

[ndantam]

I find it's usually more productive to deal with picky compiler warnings than to later search for a bug that the compiler could have warned me about.

Add the following to configure.ac:

# Enable maximum warnings
AX_CFLAGS_WARN_ALL

dnl The AX_CHECK_COMPILER_FLAGS maco was renamed to AX_CHECK_COMPILE_FLAG
dnl Checking for both versions
m4_ifdef([AX_CHECK_COMPILE_FLAG],
         [AC_DEFUN([APPEND_FLAG],
                   [AX_CHECK_COMPILE_FLAG([$1], [CFLAGS="$1 $CFLAGS"])])])

m4_ifdef([AX_CHECK_COMPILER_FLAGS],
         [AC_DEFUN([APPEND_FLAG],
                   [AX_CHECK_COMPILER_FLAGS([$1], [CFLAGS="$1 $CFLAGS"])])])

dnl If we found the flag checking macro, check some flags
m4_ifdef([APPEND_FLAG],
         [APPEND_FLAG([-Wextra])
          APPEND_FLAG([-Wconversion])
          APPEND_FLAG([-Wpointer-arith])
          APPEND_FLAG([-Wfloat-equal])
          APPEND_FLAG([-Wshadow])
          APPEND_FLAG([-Wwrite-strings])
          APPEND_FLAG([-Wc++-compat])])