CheckTypeSize: Add support for C++

Previously if headers required to check the size of a type can be
compiled with C++ compiler only, the check would fail because the C
compiler fails.  As a consequence, HAVE_${VARIABLE} would be set to
false, and ${VARIABLE} will be empty.

Teach CHECK_TYPE_SIZE to accept a new optional argument LANGUAGE
that allows one to explicitly set the compiler to use.  The new
signature is therefore:

   CHECK_TYPE_SIZE(TYPE VARIABLE [BUILTIN_TYPES_ONLY]
                                 [LANGUAGE <language>])
CheckTypeSize: Add unit tests
Merge topic 'CheckTypeSize_CXX' into next

e6cec64 CheckTypeSize: Add unit tests
07a2342 CheckTypeSize: Add support for C++