flex: fix m4 issue on target

flex: fix m4 issue on target

Flex needs m4 to run (see below) and, since the create_wrapper
introduces a bash dependency on target, give the path to m4 binary in
the configure command line.

Snippet from the flex documentation:
"The macro processor m4 must be installed wherever flex is installed.
<...>
m4 is only required at the time you run flex."

[YOCTO #5329]

(From OE-Core rev: 64030f37b34f75144f53eef42d5822ede79e08bd)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>