regen src.conf.5 for clang-4.0.0 merge

Note that makeman's use of 'make showconfig' interacts poorly with
the COMPILER_FEATURES test in share/mk/src.opts.mk, because it tests the
host compiler, not the bootstrap compiler that will actually be used to
build world. This causes it to report that Clang is enabled by default
on MIPS and PowerPC.

For example:
% make TARGET_ARCH=mips64 showconfig | grep CLANG
MK_CLANG = yes
MK_CLANG_BOOTSTRAP = no
MK_CLANG_EXTRAS = no
MK_CLANG_FULL = yes
MK_CLANG_IS_CC = no

I am committing this version anyway to avoid extraneous diffs in
src.conf.5 after every other WITH_/WITHOUT_FOO change.

In addition, we intend to switch to a C++11 compiler for all archs for
12.0 (either by fixing Clang for those archs, or by requiring an
external toolchain), and then src.conf.5 will be correct.

Sponsored by: The FreeBSD Foundation