package/spidermonkey: depends on host gcc >= 4.9

If GCC on the host is older than 4.9, during the configure stage an
error is thrown such as:

checking whether the host C compiler can be used... no
ERROR: Only GCC 4.9 or newer is supported (found version 4.8.2).

Depend on the host gcc version being >= 4.9

Fixes:
http://autobuild.buildroot.net/results/192661452fd522bf46b23d5a7a522eb853a984bf



Signed-off-by: Adam Duskett <aduskett@greenlots.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>