AM_PROG_FLEX macro will set the LEX variable using the missing
script when the flex is not present. This will confuse the
configure.ac check, which expects the AC_PROG_FLEX behaviour,
and will so fail to detect the missing flex:

	AS_IF([test x"$LEX" != "x:" -a x"$YACC" != xyacc],
		[enable_assembler=yes],
		[enable_assembler=no])

This is because AM_PROG_LEX sets the LEX variable to
"${SHELL} /home/sc/intel-gpu-tools/build-aux/missing flex",
while AC_PROG_LEX would set it to ":".

If for some reason we really need to keep AM_PROG_LEX,
alternative fix could be something like this placed before
the above AS_IF check:

	AC_MSG_CHECKING([checking for working flex])
	if ! eval "$LEX --version >/dev/null 2>&1"; then
		AC_MSG_RESULT([failed])
		LEX=:
	else
		AC_MSG_RESULT([pass])
	fi

Note the evil eval needed to recursively expand variables.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>