Is building with tcmalloc supported?

[mbroadst]

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 and FOLLY_ASSUME_NO_TCMALLOC 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!