perf tools: Make GTK2 support opt-in

This is bitrotting, nobody is stepping up to work on it, and since we
treat warnings as errors, feature detection is failing in its main,
faster test (tools/build/feature/test-all.c) because of the GTK+2
infobar check.

So make this opt-in, at some point ditch this if nobody volunteers to
take care of this.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>