configure: add missing dependency to libm.
configure: add missing dependency to libm.

Build broke when trying to compile with expressive debug CFLAGS (-g3).
This was root-caused to the lack of the "-lm" linker flag. By adding a
simple autoconf check we ensure that libm is linked.

More specifically, recent VEBOX changes depend on cos() and sin() math
functions.

Signed-off-by: Joe Konno <a81ddd8a3c0bd1583a333acdc9e7599f7063b6e9@intel.com>

halleyzhao/libva-intel-driver-fdo,uartie/vaapi-intel-driver,uartie/vaapi-intel-driver,chenxianqin/vaapi-intel-driver,halleyzhao/libva-intel-driver-fdo,01org/iotg-lin-gfx-va-driver,chenxianqin/vaapi-intel-driver,chenxianqin/vaapi-intel-driver,01org/iotg-lin-gfx-va-driver,gbeauchesne/libva-intel-driver,chenxianqin/vaapi-intel-driver,halleyzhao/libva-intel-driver-fdo,gbeauchesne/libva-intel-driver,01org/iotg-lin-gfx-va-driver,halleyzhao/libva-intel-driver-fdo,uartie/vaapi-intel-driver,gbeauchesne/libva-intel-driver,chenxianqin/vaapi-intel-driver,uartie/vaapi-intel-driver,gbeauchesne/libva-intel-driver,gbeauchesne/libva-intel-driver,uartie/vaapi-intel-driver,01org/iotg-lin-gfx-va-driver,01org/iotg-lin-gfx-va-driver