With gcc 4.9, gold seems to be a bit too aggressive in its use of -Wl,--as-needed. tcmalloc is interesting since it's employed essentially via linking, and doesn't actually require calling any symbols in the library directly.

Here's a minimal example:

$ echo 'int main() { void *x = malloc(10); return 0; }' | gcc -include stdlib.h -x c - -Wl,--as-needed -fuse-ld=gold -ltcmalloc
$ objdump -p a.out | grep NEEDED
NEEDED libc.so.6

Removing one of "-Wl,--as-needed" or "-fuse-ld=gold" will result in libtcmalloc being linked. You can further tell that libtcmalloc is in use because valgrind will always report 0 heap usage for said binary.