Fix builds failing on BSD-like (such as OSX) due to an irrlicht bug (temporary fix)

Details:
- https://sourceforge.net/p/irrlicht/bugs/433/
- https://github.com/minetest/minetest/issues/1687#issuecomment-61368769
- https://forum.minetest.net/viewtopic.php?f=42&t=9190&start=125#p159364

In case when "settings.h" is included from "emerge.cpp" or
"environment.cpp", u64 type has "unsigned long" length because
previously <stdint> was included. When "settings.h" is included from
"settings.cpp", u64 has "unsigned long long" length because no <stdint>
was included previously. This leads to different signatures of "setU64" method
and linker cannot find appropriate symbol.

The best fix of this bug should be done in the Irrlicht, but as hotfix I
think this is OK and better than types changing.

Previously this bug didn't appear because there was no "settings.cpp" file and
implementation of all methods was done in the header file.

src/irrlichttypes.h

@@ -20,6 +20,17 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #ifndef IRRLICHTTYPES_HEADER
 #define IRRLICHTTYPES_HEADER
 
+/* Ensure that <stdint.h> is included before <irrTypes.h>, unless building on
+ * MSVC, to address an irrlicht issue: https://sourceforge.net/p/irrlicht/bugs/433/
+ *
+ * TODO: Decide whether or not we support non-compliant C++ compilers like old
+ *       versions of MSCV.  If we do not then <stdint.h> can always be included
+ *       regardless of the compiler.
+ */
+#ifndef _MSC_VER
+#	include <stdint.h>
+#endif
+
 #include <irrTypes.h>
 
 using namespace irr;
@@ -32,11 +43,9 @@ using namespace irr;
 	typedef unsigned long long u64;
 #else
 	// Posix
-	#include <stdint.h>
 	typedef int64_t s64;
 	typedef uint64_t u64;
 #endif
 #endif
 
 #endif
-