Fix Windows MSYS2 build (mostly)

Outputs/OpenGL/OpenGL.hpp

@@ -23,6 +23,7 @@
 #else
 #define GL_GLEXT_PROTOTYPES
 #include <GL/gl.h>
+#include <GL/glext.h>
 #endif
 
 #ifndef NDEBUG

Outputs/OpenGL/ScanTargetGLSLFragments.cpp

@@ -10,6 +10,10 @@
 
 #include <cmath>
 
+#ifndef M_PI
+#define M_PI 3.1415926f
+#endif
+
 using namespace Outputs::Display::OpenGL;
 
 // MARK: - State setup for compiled shaders.

SignalProcessing/FIRFilter.cpp

@@ -9,6 +9,10 @@
 #include "FIRFilter.hpp"
 #include <cmath>
 
+#ifndef M_PI
+#define M_PI 3.1415926f
+#endif
+
 using namespace SignalProcessing;
 
 /*

Storage/MassStorage/SCSI/Target.hpp

@@ -279,7 +279,7 @@ struct Executor {
 
 		auto copy_string = [] (uint8_t *destination, const char *source, size_t length) -> void {
 			// Copy as much of the string as will fit, and pad with spaces.
-			uint8_t *end = reinterpret_cast<uint8_t *>(stpncpy(reinterpret_cast<char *>(destination), source, length));
+			uint8_t *end = reinterpret_cast<uint8_t *>(strncpy(reinterpret_cast<char *>(destination), source, length));
 			while(end < destination + length) {
 				*end = ' ';
 				++end;