Add support for compiling with external TomsFastMath library provided by
the system instead of compiling the vendored copy into libclamav.
The vendored source is still built directly into libclamav instead of as
a separate library the way libmspack is done.
The rationale is that:
A) it's more complicated to deal with possibly compiling as static or
dynamic, and also
B) libmspack and libunrar are compiled separately primarily because of
licensing concerns. TomsFastMath public domain, so that isn't a concern.
Resolves: https://bugzilla.clamav.net/show_bug.cgi?id=12562
