diff --git a/src/common.h b/src/common.h
index 8759d7d03b..2c3fb763a7 100644
--- a/src/common.h
+++ b/src/common.h
@@ -78,7 +78,7 @@
 	#ifndef FASTCALL
 		#ifdef __GNUC__
 			#define FASTCALL __attribute__((fastcall))
-		#elif _MSC_VER
+		#elif defined(_MSC_VER)
 			#define FASTCALL __fastcall
 		#else
 			#pragma message "Not using fastcall calling convention, please check your compiler support"
diff --git a/src/rct2.h b/src/rct2.h
index 1d12df5cd1..ce54a41303 100644
--- a/src/rct2.h
+++ b/src/rct2.h
@@ -131,7 +131,7 @@ typedef uint8 colour_t;
 
 #endif // defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 
-#if !(_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)
+#if !((defined (_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200809L) || (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE >= 700))
 	char *strndup(const char *src, size_t size);
 #endif // !(POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)