Don't use check_cxx_source_runs to make cross-compiling easier

[russelltg]

Description

Fix #3773

Tests

I did test it on my local Linux system, and it properly detects that it doesn't need to link against . I also tested compiling the test source file in a system that needs linking against latomic (32-bit ubuntu 14.04 using clang, there is probably some other system that is this way as well) and the compile itsself did indeed fail with undefined reference errors, as expected. So this check should be just as robust.

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate).
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

I see. I think I assumed that check_cxx_source_compiles only checked for compilation of the source file, and did not realize it would link the executable. You're right, that's enough, it doesn't need to be executed. LGTM.