Log Message:
Update to 0.39.3

  mkc_check_common.sh: do not hide stderr in function "find_n_match"
  used by MKC_CHECK_PROGS. As a result stderr of mkc_which and
  program we search for will be a part of
  _mkc_{custom_,}prog_XXX.res. This helps debugging.

  Before using gmake in mk/mkc_imp.foreign_autotools.mk and
  builtins/prog_gmake always unset environment variable MAKEFLAGS
  because bmake and gmake are incompatible.

  tests/WARNERR: improve test for clang-16.0.6 that does not warn
  about sign/unsigned pointer mistype