flex: prevent flex binary to be built

Since we prevent the flex binary to be built, we also need to prevent
the documentation to be built, otherwise it will fail like this:

Making all in doc
make[2]: Entering directory '/br/output/build/flex-2.6.4/doc'
make[2]: *** No rule to make target '../src/flex', needed by 'flex.1'.
Stop.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>

diff -rup a/Makefile.in b/Makefile.in
--- a/Makefile.in	2017-05-06 15:49:09.000000000 +0100
+++ b/Makefile.in	2017-05-10 14:35:20.152794902 +0100
@@ -450,7 +450,6 @@ EXTRA_DIST = \
 
 SUBDIRS = \
 	src \
-	doc \
 	examples \
 	po \
 	tests \
diff -rup a/src/Makefile.in b/src/Makefile.in
--- a/src/Makefile.in	2017-05-06 15:49:09.000000000 +0100
+++ b/src/Makefile.in	2017-05-10 14:35:01.918464036 +0100
@@ -90,7 +90,7 @@ PRE_UNINSTALL = :
 POST_UNINSTALL = :
 build_triplet = @build@
 host_triplet = @host@
-bin_PROGRAMS = flex$(EXEEXT)
+bin_PROGRAMS =
 @ENABLE_BOOTSTRAP_TRUE@noinst_PROGRAMS = stage1flex$(EXEEXT)
 @CROSS_TRUE@am__append_1 = \
 @CROSS_TRUE@		      ../lib/malloc.c \