package/x11r7/xdriver_xf86-video-intel: use -O2 build optimisation level

Reported by Gustavo:
http://lists.busybox.net/pipermail/buildroot/2016-August/169959.html

The fix was suggested on the Gentoo bugtracker:
https://bugs.gentoo.org/show_bug.cgi?id=582910#c4

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/x11r7/xdriver_xf86-video-intel/xdriver_xf86-video-intel.mk

@@ -10,9 +10,11 @@ XDRIVER_XF86_VIDEO_INTEL_LICENSE = MIT
 XDRIVER_XF86_VIDEO_INTEL_LICENSE_FILES = COPYING
 XDRIVER_XF86_VIDEO_INTEL_AUTORECONF = YES
 
-# this fixes a getline-related compilation error in src/sna/kgem.c
+# -D_GNU_SOURCE fixes a getline-related compile error in src/sna/kgem.c
+# We force -O2 regardless of the optimization level chosen by the user,
+# as compiling this package is known to be broken with -Os.
 XDRIVER_XF86_VIDEO_INTEL_CONF_ENV = \
-	CFLAGS="$(TARGET_CFLAGS) -D_GNU_SOURCE"
+	CFLAGS="$(TARGET_CFLAGS) -D_GNU_SOURCE -O2"
 
 XDRIVER_XF86_VIDEO_INTEL_CONF_OPTS = \
 	--disable-xvmc \