Add support for breaking into debugger for Linux

Use Linux-specific /proc/$PID/status file to check whether we're being
debugged and a generic raise(SIGTRAP) to actually break into the debugger.

include/internal/catch_platform.h

@@ -15,6 +15,8 @@
 #define CATCH_PLATFORM_IPHONE
 #elif defined(WIN32) || defined(__WIN32__) || defined(_WIN32) || defined(_MSC_VER)
 #define CATCH_PLATFORM_WINDOWS
+#elif defined(linux) || defined(__linux) || defined(__linux__)
+#define CATCH_PLATFORM_LINUX
 #endif
 
 #endif // TWOBLUECUBES_CATCH_PLATFORM_H_INCLUDED