glew: fix Makefile race

Fix a Makefile race resulting in the target creating a directory being
executed after the target to write into that directory.

[ YOCTO #14485 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>