Ignore `ax_cxx_compile_stdcxx.m4` when running `make check-style`
Ignore `ax_cxx_compile_stdcxx.m4` when running `make check-style`

`ax_cxx_compile_stdcxx.m4` currently fails `make check-style` due to
line lengths. Ignore the style issues though, given that it's a contributed
m4 file.

Signed-off-by: Enji Cooper <d6bb01297bdcc8d1a5e14be12c1049fd9e385b00@gmail.com>

jmmv/kyua,jmmv/kyua,jmmv/kyua,jmmv/kyua