Is building with tcmalloc supported?

Hi, I spent a few hours vendoring folly into a codebase using tcmalloc the other day and ran into a few issues. I noticed reference to [USE_TCMALLOC](https://github.com/facebook/folly/blob/1de3e85fd83b45541903cebe997da2f69e68a93d/folly/memory/Malloc.h#L230) and [FOLLY_ASSUME_NO_TCMALLOC](https://github.com/facebook/folly/blob/1de3e85fd83b45541903cebe997da2f69e68a93d/folly/memory/Malloc.h#L225C13-L225C37) but neither of those are set in any of the build files I looked at. Once actually compiling I ran into a number of issues with symbol redefinitions (`nallocx`, `sdallocx` and `MallocExtension_Internal_GetNumericProperty`) as well as missing symbols (`mallctl` and friends). Do y'all support building with tcmalloc? If not, would you accept patches to (re)introduce support? Thank you!