Add AX_CHECK_COMPILE_FLAG (from Autotools packages)

It's fix the build with Clang and --enable-werror, don't support -WClobberred and display error about unknown warning option
error: unknown warning option '-Wclobbered'; did you mean '-Wconsumed'? [-Werror,-Wunknown-warning-option]
Add some news CFLAGS:
* -Wpragmas
* -Wunreachable-code
* -Wpedantic
* -Waddress
* -Wattributes
* -Wdiv-by-zero
* -Wheader-guard
Merge branch 'add_check_compile_flag' of https://github.com/alagoutte/nghttp2 into alagoutte-add_check_compile_flag
Fix compile error with clang -Wunreachable-code