perf tools: Allow external definition of flex and bison binary names

Allow user to define flex and bison binary names by passing FLEX and
BISON variables.
Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Paul Turner <pjt@google.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20170827075442.108534-3-davidcc@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.perf

@@ -164,8 +164,8 @@ LN      = ln -f
 MKDIR   = mkdir
 FIND    = find
 INSTALL = install
-FLEX    = flex
-BISON   = bison
+FLEX    ?= flex
+BISON   ?= bison
 STRIP   = strip
 AWK     = awk