build(linux): remove GCC 4.9 checks
build(linux): remove GCC 4.9 checks

The CI workers now use GCC 7.5 for everything. Users that need to build
on GCC 4.8 will have to disable rpmalloc/xxhash, so the 4.9 checks are
not useful anymore.

TheMrMilchmann/lwjgl3,LWJGL-CI/lwjgl3,LWJGL/lwjgl3,code-disaster/lwjgl3,LWJGL/lwjgl3,TheMrMilchmann/lwjgl3,LWJGL-CI/lwjgl3,code-disaster/lwjgl3,LWJGL/lwjgl3,TheMrMilchmann/lwjgl3,LWJGL-CI/lwjgl3,LWJGL/lwjgl3,code-disaster/lwjgl3,LWJGL-CI/lwjgl3,TheMrMilchmann/lwjgl3,code-disaster/lwjgl3