message: build: force gperftools to link with pthread.
  This seems necessary for some link combinations with libunwind and clang. I have an open thread
  internally to figure out why gperftools doesn't auto-detect and why it is version dependent.
diff: @@ -3,5 +3,5 @@ set -e
  wget https://github.com/gperftools/gperftools/releases/download/gperftools-2.5/gperftools-2.5.tar.gz
  tar xf gperftools-2.5.tar.gz
  cd gperftools-2.5
  -./configure --prefix=$THIRDPARTY_BUILD --enable-shared=no --enable-frame-pointers
  +LDFLAGS="-lpthread" ./configure --prefix=$THIRDPARTY_BUILD --enable-shared=no --enable-frame-pointers
  make install
  