Add -lm to library list for plugins

The tuner uses the log2 function that is defined in libm, but we
were relying on someone else to pull in libm, which is technically
wrong.  Checking for what library provides libm fixes this issue,
with the side effect of adding a -lm to the link line for the
libnccl-net plugin.  This is not strictly needed, but linkers also
will do the right thing most of the time and not actually include
that dependency in the list of libraries that must be loaded for
the plugin.  So this is a lot less configure code compared to
each plugin having its own LIBS list, for the same outcome.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

configure.ac

@@ -62,6 +62,7 @@ AC_CHECK_FUNC([memset], [], [AC_MSG_ERROR([NCCL OFI Plugin requires memset funct
 AC_CHECK_FUNC([realpath], [], [AC_MSG_ERROR([NCCL OFI Plugin requires realpath function.])])
 AC_SEARCH_LIBS([dlopen], [dl], [], [AC_MSG_ERROR([NCCL OFI Plugin requires dlopen])])
 AC_SEARCH_LIBS([pthread_getspecific], [pthread], [], [AC_MSG_ERROR([NCCL OFI Plugin requires pthreads.])])
+AC_SEARCH_LIBS([log2], [m], [], [AC_MSG_ERROR([NCCL OFI Plugin requires the log2 library function.])])
 
 # Check for GCC builtin functions
 CHECK_GCC_BUILTIN([__builtin_expect])