Report compiler in -d -bV. Clang compat.

Exim successfully builds with clang, albeit with a number of warnings.

 * Our %n usage in printf() calls appears to be correct and safe, AFAICT.
 * dummy functions are, unsurprisingly, unused
 * Valgrind macros cause vociferous complaints
 * Dynamic modules *not* tested

Further clang testing on my part will require an OS update and clang
2.9 to get -rdynamic support.