AX_CHECK_X86_FEATURES for CXXFLAGS

[The autoconf AX_CHECK_X86_FEATURES](https://www.gnu.org/software/autoconf-archive/ax_check_x86_features.html) will test for -mavx, -mavx2, etc and add it to CFLAGS. What is the right way to propagate it to CXXFLAGS too? Is it "clean" to always add CFLAGS to CXXFLAGS? It doesn't seem so... But my C++ code contains some avx2 code, so I need this.