Work around autotools setting C11 standard in CC/CXX

In autoconf >=2.70, C11 is set by default for $CC and $CXX via the
-std=...11 flag. In this patch, we split the "-std" flag out of the $CC
and $CXX variables, which we traditionally assume to be just the
executable name/path, and move it to $CFLAGS/$CXXFLAGS instead.

Fixes #24324