C++17 support for DoD:S Linux

[APGRoboCop]

I wasn't sure if you knew this, but HL2SDK-DODS appears to have a depreciated register in mathlib.h, maybe more, as I cannot compile RCBot2 for DoDS Linux:

/home/linux/Dropbox/src/rcbot2/alliedmodders/hl2sdk-dods/public/mathlib/mathlib.h:331:2: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister] register double __cosr, __sinr; ^~~~~~~~~ /home/linux/Dropbox/src/rcbot2/alliedmodders/hl2sdk-dods/public/mathlib/mathlib.h:331:2: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister] register double __cosr, __sinr; ^~~~~~~~~
Of course, this doesn't affect Windows as it only affects Linux:
#elif defined( _LINUX ) || defined ( __APPLE__ ) register double __cosr, __sinr; __asm __volatile__ ("fsincos" : "=t" (__cosr), "=u" (__sinr) : "0" (radians)); *sine = __sinr; *cosine = __cosr; #endif

I would use the -Wno-register Clang flag in my AMBuildScript, but I don't think that would be wise, as suppressing the warning does not fix the underlying issue.