enable tcmalloc

I think we should switch to using tcmalloc [1]. Until we fix Bug #1641360, tcmalloc will help because it can isolate the engine thread's malloc'ing from the rest of Mixxx's threads because each thread gets its own thread-local memory pool which only locks a global mutex when it needs to increase the size of its pool.

It's a trivial change code-wise (we already have a build option for it) we just need to add it to the build environments.

[1] https://gperftools.github.io/gperftools/tcmalloc.html

https://github.com/gperftools/gperftools