tools: flex: fix build with automake 1.15.1

After the update of GNU automake, the flex configure fails with

    configure.ac:31: error: version mismatch.  This is Automake 1.15.1,
    configure.ac:31: but the definition used by this AM_INIT_AUTOMAKE
    configure.ac:31: comes from Automake 1.15.

Apply the autoreconf host fixup to properly regenerate configure before
to avoid triggering the version mismatch. Also refresh the patch while
we're at it.

Fixes FS#885.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

tools/flex/patches/100-disable-tests-docs.patch

@@ -1,8 +1,8 @@
 --- a/Makefile.am
 +++ b/Makefile.am
-@@ -45,10 +45,7 @@ EXTRA_DIST = \
+@@ -43,10 +43,7 @@ EXTRA_DIST = \
+ 
  SUBDIRS = \
- 	lib \
  	src \
 -	doc \
 -	examples \