syslibroot for macOS needs to be manually fixed also for GCC v5

[giordano]

Fix error reported in #381 (comment), observed in JuliaPackaging/Yggdrasil#421.

[staticfloat]

Can we get a MWE for something that requires the syslibroot set like this? I want to run this against all the platforms and see if it's actually needed for more than just GCC 4/5. And I'd like to set up a test so it doesn't bitrot. :)

[giordano]

HelloWorldFortran reproduces the error when building for x86_64-apple-darwin14-libgfortran3 with GCC v5

[staticfloat]

Okay so it looks like we dodge this in the default testsuite but not in HelloWorldFortran because in the default testsuite, we actually run $(FC) -c -o blah.o blah.f and then run $(CC) -o blah blah.o, so the thing doing the linking is $(CC) which is clang. Whereas in HelloWorldFortran, we directly run $(FC) -o blah blah.f, which means that it's actually gfortran doing the linking.

I will update the test suite to compile HelloWorldFortran twice so we can exercise both codepaths.

[staticfloat]

I've got a slightly different testing approach bundled in with #640, so once that goes green we can just merge that. Thanks for the find!