After r232322, it turned out many people (and some ports) are building
kernel modules using their old installed /usr/share/mk/bsd.*.mk files,
instead of the updated ones in their source tree. This leads to errors
like:

"sys/conf/kmod.mk", line 111: Malformed conditional (${MK_CLANG_IS_CC} == "no" && ${CC:T:Mclang} != "clang")

Obviously, these errors will go away after a "make installworld", or
alternatively, by using "make buildenv" before attempting to manually
build modules.

However, since it is apparently an expected use case to build using old
.mk files, change the way we test for clang, so it also works when the
MK_CLANG_IS_CC macro doesn't exist.

Note the conditional expressions are becoming rather unreadable now, but
I will attempt to fix that on a followup commit.

MFC after: 2 weeks