message: build: fix gperftools build on OS X
  gperftools-2.6.1 doesn't build on OS X without adding -D_XOPEN_SOURCE=1 to enable some
  deprecated libc code.
diff: @@ -7,5 +7,12 @@ VERSION=2.6.1
  wget -O gperftools-$VERSION.tar.gz https://github.com/gperftools/gperftools/releases/download/gperftools-$VERSION/gperftools-$VERSION.tar.gz
  tar xf gperftools-$VERSION.tar.gz
  cd gperftools-$VERSION
  +
  +# TODO(zuercher): Remove this workaround for https://github.com/gperftools/gperftools/issues/910
  +if [[ `uname` == "Darwin" ]];
  +then
  + export CPPFLAGS="$CPPFLAGS -D_XOPEN_SOURCE=500 -D_DARWIN_C_SOURCE"
  +fi
  +
  LDFLAGS="-lpthread" ./configure --prefix=$THIRDPARTY_BUILD --enable-shared=no --enable-frame-pointers --disable-libunwind
  make V=1 install
  