Build: Include unistd.h where required

Some modules require an include of unistd.h (e.g. for getcwd and isatty
definitions).

These changes were required to build successfully on a Fedora 17 system,
using GCC 4.7.0 & glibc-headers 2.15.

ispc.cpp

@@ -44,6 +44,8 @@
 #include <windows.h>
 #include <direct.h>
 #define strcasecmp stricmp
+#else
+#include <unistd.h>
 #endif
 #include <llvm/LLVMContext.h>
 #include <llvm/Module.h>