perf tools: Check for flex and bison before continuing building

Check whether both executables are present on the system before
continuing with the build instead of failing halfway, if either are
missing.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1359979554-9160-1-git-send-email-bp@alien8.de
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>