configure.ac: Add more warning flags (v3)

v3: Turn check for  -Wint-to-pointer-cast into check for
    -Werror=int-to-pointer-cast

Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

configure.ac

@@ -29,9 +29,11 @@ AC_CHECK_PROGS([PYTHON], [python2 python python3])
 AX_CODE_COVERAGE
 
 AX_CHECK_COMPILE_FLAG([-Wall],                                 [CFLAGS="$CFLAGS -Wall"])
+AX_CHECK_COMPILE_FLAG([-Wextra],                               [CFLAGS="$CFLAGS -Wextra"])
 AX_CHECK_COMPILE_FLAG([-Werror=implicit-function-declaration], [CFLAGS="$CFLAGS -Werror=implicit-function-declaration"])
 AX_CHECK_COMPILE_FLAG([-Werror=missing-prototypes],            [CFLAGS="$CFLAGS -Werror=missing-prototypes"])
 AX_CHECK_COMPILE_FLAG([-Wmissing-prototypes],                  [CFLAGS="$CFLAGS -Wmissing-prototypes"])
+AX_CHECK_COMPILE_FLAG([-Werror=int-to-pointer-cast],           [CFLAGS="$CFLAGS -Werror=int-to-pointer-cast"])
 
 AC_MSG_CHECKING([for native Win32])
 case "$host_os" in