cpu.cmake: fix compiler flag detection w/3.17.0+
cpu.cmake: fix compiler flag detection w/3.17.0+

Between 3.17.0 and 3.18.2 check_cxx_compiler_flag() sets a normal
variable at parent scope while check_cxx_source_compiles() continues to
set an internal cache variable, so we unset both to avoid the failure /
success state persisting between checks.

This was fixed in 3.18.3 [1], but regressed shortly afterward [2];
currently seen with 3.20.0 installed via homebrew.

[1] https://gitlab.kitware.com/cmake/cmake/-/issues/21207
[2]
90dead024c CheckCompilerFlag: unified way to check compiler flags per language

Change-Id: I57282f89b07a9cfd85aca7569380f7d115c0b3cf

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp