MFC r293014:

Merge r293006 from clang380-import branch:

For determining the compiler version, quote the string to be echo'd,
otherwise the command might fail. This is because clang -v now results
in the following:

FreeBSD clang version 3.8.0 (trunk 256633) (based on LLVM 3.8.0svn)

The second "3.8.8svn)" string tripped up the shell command.

MFC r265829 (by imp):
Support, to the extent we generate proper command lines, compiling
with clang 3.3. Useful for test building -current on a -stable system
in individual directories. Potentially useful if we ever want to
support, say, gcc 4.8 or 4.9's new warnings when building with an
external toolchain (but such support not yet committed). Document
the bsd.compiler.mk interface.

MFC r266587 (by imp):
Allow CC to not actually exist. During the ports INDEX run, all the
Makefiles are evaluated without building things. In a normal build,
the prerequisites would be built, and CC would be an actual thing. In
an INDEX build, though, they don't exists. Redirect stderr to get rid
of annoying messages, and assume that the compiler version is 0 if the
actual compiler can't tell us. Do this in preference to guessing based
on numbers because gcc410 might be 4.10, or 4.1.0 and without
carefully crafted special knowledge we differentiate between them
easily (also ming-gcc has no clues at all). Elsewhere, don't trust
the compiler version if it is 0.

MFC r273405 (by bapt):
When using an external toolchain note that gcc 4.8+ supports C++11

Submitted by: imp

MFC r275557 (by ngie):
Fix typos in comments and wrap to <80 columns

MFC r275588 (by ngie):
${CC} --version doesn't need to be queried if both COMPILER_TYPE and
COMPILER_VERSION are known

MFC r275589 (by ngie):
The previous commit should have been a logical or not a logical "and"

Pointyhat to: me

MFC r265829 (by imp):
Support, to the extent we generate proper command lines, compiling
with clang 3.3. Useful for test building -current on a -stable system
in individual directories. Potentially useful if we ever want to
support, say, gcc 4.8 or 4.9's new warnings when building with an
external toolchain (but such support not yet committed). Document
the bsd.compiler.mk interface.

MFC r266587 (by imp):
Allow CC to not actually exist. During the ports INDEX run, all the
Makefiles are evaluated without building things. In a normal build,
the prerequisites would be built, and CC would be an actual thing. In
an INDEX build, though, they don't exists. Redirect stderr to get rid
of annoying messages, and assume that the compiler version is 0 if the
actual compiler can't tell us. Do this in preference to guessing based
on numbers because gcc410 might be 4.10, or 4.1.0 and without
carefully crafted special knowledge we differentiate between them
easily (also ming-gcc has no clues at all). Elsewhere, don't trust
the compiler version if it is 0.

MFC r273405 (by bapt):
When using an external toolchain note that gcc 4.8+ supports C++11

Submitted by: imp

MFC r275557 (by ngie):
Fix typos in comments and wrap to <80 columns

MFC r275588 (by ngie):
${CC} --version doesn't need to be queried if both COMPILER_TYPE and
COMPILER_VERSION are known

MFC r275589 (by ngie):
The previous commit should have been a logical or not a logical "and"

Pointyhat to: me