support building with VS2013

From bruen...@google.com on February 25, 2014 14:33:05

A quick try hits this:

D:\derek\dr\git\src\core\x86\decode.c : fatal error C1041: cannot open progr am database 'd:\derek\dr\build_suite\build_debug-i32-static-ext\lib32\drdecode.pdb': if multiple CL.EXE write to the same .PDB file, please use /FS

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1376