Rewrite find_cxx_header config configure.ac

Rewrite find_cxx_header config configure.ac

With clang if the main file is <stdin>, the first non-main-file
dependency is not listed in the -MP output.  Although it was fixed
on clang-16 [1], this change adds portability for older version.

[1] https://github.com/llvm/llvm-project/commit/ff9576f74514b836e1ba0268409a2ecb919d7118

Reviewed-by: Fangrui Song <maskray@google.com>