Fix vbsp missing skybox error case when building cubemaps.

The code previoused checked the address of a stack array that would always
evaluate non-zero, which wasn't what was intended.
Fix SortEntry operator< not being const.

This solves a problem where the standard library expects it to be const,
which causes compilation problems on GNU/Linux.
Fix bad class declaration in utils/vbsp/ivp.

The duplicated namespace in the member declaration causes the class to not
compile using gcc.
Fix use of temporary vector in utils/vrad/vraddetailprops.cpp.

This fixes a gcc compile error.
Avoid conflict with gamma(3) in utils/vrad/vrad.h.

The name of the global variable conflicts with the deprecated gamma function
from <math.h> on some systems. Additionally the variable appears to be
unused - should it rather just be deleted from the codebase?
Use Q_strncpy in utils/vbsp/detailobjects.cpp.
Fix bsp lump names being declared non-portably.

gcc issues warnings when encountering multicharacter constants, so we'll
simply emulate them in a manner works portably. The code assumes we are
using a little-endian machine - the real solution would be using using four
chars instead of an enum.
Merge commit '82fe37f'