package/libdrm: fix building if GCC's C standard is not gnu99

Fixes
checking for /home/fli4l/buildroot/output/host/usr/bin/i586-buildroot-linux-uclibc-gcc option to accept ISO C99... unsupported
configure: error: Building libdrm requires C99 enabled compiler

using this defconfig
BR2_KERNEL_HEADERS_4_0=y
BR2_BINUTILS_VERSION_2_25=y
BR2_TOOLCHAIN_BUILDROOT_CXX=y
BR2_PACKAGE_LIBDRM=y

Patch inspired by
http://git.buildroot.net/buildroot/commit/?id=5cf5b390385fb6325485e37dc9d38e1e3ac1f091

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libdrm/libdrm.mk

@@ -19,6 +19,8 @@ LIBDRM_CONF_OPTS = \
 	--disable-cairo-tests \
 	--disable-manpages
 
+LIBDRM_CONF_ENV = ac_cv_prog_cc_c99='-std=gnu99'
+
 ifeq ($(BR2_PACKAGE_LIBDRM_INTEL),y)
 LIBDRM_CONF_OPTS += --enable-intel
 LIBDRM_DEPENDENCIES += libatomic_ops libpciaccess