MFC r303841,r312012,r312213:

r303841 (by bdrewery):

Revert r298434 which should be fixed by r301287, r301394, and r301403.

PR: 208703, 208963

r312012:

fmaxmin_test still fails with clang 3.9.x.. bypass the test

PR: 208703

r312213:

Turn COMPILER_VERSION/COMPILER_TYPE make check into a compile-time check
of the clang version

This works around breakage on ^/stable/10 when running installworld from
a ^/stable/10 host where the test wouldn't be compiled on the first
go-around and would be missing when make installworld is run.

PR: 208703