Gperftools target always rebuilds.

From make --debug=v:

Considering target file `gperftools-2.0/libprofiler.la'.
Considering target file `gperftools-2.0-stamp'.
Considering target file `gperftools-2.0.tar.gz'.
Finished prerequisites of target file `gperftools-2.0.tar.gz'.
No need to remake target `gperftools-2.0.tar.gz'; using VPATH name `../../third_party/gperftools-2.0.tar.gz'.
Finished prerequisites of target file `gperftools-2.0-stamp'.
Prerequisite `../../third_party/gperftools-2.0.tar.gz' is older than target `gperftools-2.0-stamp'.
No need to remake target `gperftools-2.0-stamp'.
Finished prerequisites of target file `gperftools-2.0/libprofiler.la'.
Prerequisite `gperftools-2.0-stamp' is newer than target `gperftools-2.0/libprofiler.la'.
Must remake target `gperftools-2.0/libprofiler.la'.
cd gperftools-2.0 && ./configure --enable-shared=no --with-pic --srcdir=. && \
make
Commands of `gperftools-2.0/libprofiler.la' are being run.

I will look at adding a build-stamp target similar to what we have for protobuf. (see: third_party/Makefile.am)