[BUG] Unneccesary re-run of cxx_feature_check on cmake config re-run

[theComputeKid]

It is not necessary to run cxx_feature_check for HAVE_GNU_POSIX_REGEX, HAVE_POSIX_REGEX, HAVE_STEADY_CLOCK or HAVE_PTHREAD_AFFINITY on every cmake re-config.

E.g. log on cmake config:

-- Google Benchmark version: v1.9.4, normalized to 1.9.4
-- Performing Test HAVE_GNU_POSIX_REGEX -- failed to compile
-- Performing Test HAVE_POSIX_REGEX -- failed to compile
-- Performing Test HAVE_STEADY_CLOCK -- success
-- Performing Test HAVE_PTHREAD_AFFINITY -- failed to compile

No compiler tests should be performed on a re-config as it is tied to the C/CXX compiler.

The issue is because of the following code:

cxx_feature_check(STD_REGEX)
cxx_feature_check(GNU_POSIX_REGEX)
cxx_feature_check(POSIX_REGEX)
if(NOT HAVE_STD_REGEX AND NOT HAVE_GNU_POSIX_REGEX AND NOT HAVE_POSIX_REGEX)
  message(FATAL_ERROR "Failed to determine the source files for the regular expression backend")
endif()

This is not necessary. A suggestion to solve this is to change cxx_feature_check such that it sets the necessary feature variables in the cache instead of a normal variable. On config re-run, CMake will see that the variable is already set and not run the tests again.

To reproduce, run this twice and inspect the log:

cmake -B build -G Ninja -DBENCHMARK_DOWNLOAD_DEPENDENCIES=1

Expected behaviour: the second call to cmake should be a no-op and run no compiler tests, and print nothing to the console.

[LebedevRI]

Please feel free to submit a patch.

[theComputeKid]

As you wish. Please review, @LebedevRI, it was a bit difficult to parse the legacy CMake code.