For VMS, implement the possibility to choose 64-bit pointers with

different options:
"64"		The build system will choose /POINTER_SIZE=64=ARGV if
		the compiler supports it, otherwise /POINTER_SIZE=64.
"64="		The build system will force /POINTER_SIZE=64.
"64=ARGV"	The build system will force /POINTER_SIZE=64=ARGV.