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.

lex.ll

@@ -58,6 +58,8 @@ extern void RegisterDependency(const std::string &fileName);
 
 #ifdef ISPC_IS_WINDOWS
 inline int isatty(int) { return 0; }
+#else
+#include <unistd.h>
 #endif // ISPC_IS_WINDOWS
 
 static int allTokens[] = {