configure: Don't try to enable the log2 function on msvcrt

Some msvcrt versions (the static 64 bit libc in MSVC 10) have
a log2 function, but there is no declaration for it in the headers.
Therefore, the normal configure check might find it, but it can fail
during build or at runtime, depending on whether implicit function
declarations are an error or not.

Therefore simply ignore this function on this platform.

Signed-off-by: Martin Storsjö <martin@martin.st>
