CDRIVER-6175 omit empty CXX= from handshake

[kevinAlbs]

See CDRIVER-6175. Intended to remove an unnecessary CXX= from the handshake when a C++ compiler is not checked.

A wrapping driver may configure an empty string to indicate no check occurred. Prior to this PR that results in a platform string like:

                                                        vvvv
cfg=0x06235e88e9 posix=200809 stdc=199901 CC=GCC 11.4.0 CXX= CFLAGS=\"\" LDFLAGS=\"\"
                                                        ^^^^

After this PR:

cfg=0x06235e88e9 posix=200809 stdc=199901 CC=GCC 11.4.0 CFLAGS=\"\" LDFLAGS=\"\"

[eramongodb]

Prior code permitted MONGOC_CXX_COMPILER_ID to be undefined:

" CXX=" MONGOC_CXX_COMPILER_ID // -> " CXX="

This call to strlen() must either support a similar pattern:

if (0 != strlen("" MONGOC_CXX_COMPILER_ID)) { ... }

or be guarded by an ifdef:

#ifdef MONGOC_CXX_COMPILER_ID
   if (0 != strlen(MONGOC_CXX_COMPILER_ID)) { ... }
#endif

[kevinAlbs]

Updated as suggested. I expect MONGOC_CXX_COMPILER_ID is always defined after configuring mongoc-config-private.h.in. But this might avoid the risk of breaking consumers if they bring-their-own mongoc-config-private.h.

MONGOC_CXX_COMPILER_VERSION is updated for consistency.