Add OpenVR SDK to third_party

third_party/openvr/src/src/vrcommon/envvartools_public.cpp

+//========= Copyright Valve Corporation ============//
+#include "envvartools_public.h"
+#include <stdlib.h>
+
+#if defined(_WIN32)
+#include <Windows.h>
+
+#undef GetEnvironmentVariable
+#undef SetEnvironmentVariable
+#endif
+
+
+std::string GetEnvironmentVariable( const char *pchVarName )
+{
+#if defined(_WIN32)
+        char rchValue[32767]; // max size for an env var on Windows
+        DWORD cChars = GetEnvironmentVariableA( pchVarName, rchValue, sizeof( rchValue ) );
+        if( cChars == 0 )
+                return "";
+        else
+                return rchValue;
+#elif defined(POSIX)
+        char *pchValue = getenv( pchVarName );
+        if( pchValue )
+                return pchValue;
+        else
+                return "";
+#else
+#error "Unsupported Platform"
+#endif
+}
+
+
+bool SetEnvironmentVariable( const char *pchVarName, const char *pchVarValue )
+{
+#if defined(_WIN32)
+        return 0 != SetEnvironmentVariableA( pchVarName, pchVarValue );
+#elif defined(POSIX)
+        if( pchVarValue == NULL )
+                return 0 == unsetenv( pchVarName );
+        else
+                return 0 == setenv( pchVarName, pchVarValue, 1 );
+#else
+#error "Unsupported Platform"
+#endif
+}