ax_prog_flex.m4: Merge upstream OpenBSD fixes.
ax_prog_flex.m4: Merge upstream OpenBSD fixes.

Merge the following upstream autoconf-archive patches.

ax_prog_flex: change grep syntax to accept e.g. "flex.real" in case a wrapper or symlink is used.
AX_PROG_FLEX: avoid use of grep empty string escape extension (fix for OpenBSD)
AX_PROG_FLEX: Also accept gflex.

Signed-off-by: Vinson Lee <25a259f709b0f1b620605d2518271a846d8826e5@freedesktop.org>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Reviewed-by: Jonathan Gray <666d82badf37dc240bacece830d7f52411b6fee3@openbsd.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler