CMake: Link libm to test binaries if available.

If libm is available link it to test binaries which will otherwise
fail to compile. Tested on FreeBSD 13-STABLE.
Signed-off-by: Daniel Engberg <daniel.engberg.lists@pyret.net>
Signed-off-by: Ralph Giles <giles@thaumas.net>

test/CMakeLists.txt

 add_executable(vorbis_test util.c util.h write_read.c write_read.h test.c)
-target_link_libraries(vorbis_test PRIVATE Vorbis::vorbisenc)
+target_link_libraries(vorbis_test PRIVATE Vorbis::vorbisenc $<$<BOOL:${HAVE_LIBM}>:m>)
 add_test(NAME vorbis_test COMMAND vorbis_test)