Rename 'Standard Gaming Platform' to 'sgp' (#246)

* rename 'Standard Gaming Platform' to 'sgp'

* Adapt CMakeLists.txt files to renamed SGP directory

* Lowercase it

sgp/sgp.h

@@ -0,0 +1,30 @@
+#ifndef __SGP_
+#define __SGP_
+
+#include "local.h"
+#include "types.h"
+#include "timer.h"
+#include "debug.h"
+
+#include "video.h"
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern BOOLEAN		gfProgramIsRunning; // Turn this to FALSE to exit program
+extern CHAR8			gzCommandLine[100];	// Command line given
+extern UINT8			gbPixelDepth;				// GLOBAL RUN-TIME SETTINGS
+extern BOOLEAN		gfDontUseDDBlits;		// GLOBAL FOR USE OF DD BLITTING
+
+
+// function prototypes
+void SGPExit(void);
+void ShutdownWithErrorBox(CHAR8 *pcMessage);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif