[cmake] gcc.cmake: Enable C++ assertions and debug mode, when buildin…

…g for Debug.

Assertions are enabled for libstdc++ (gcc) and libc++ (llvm/clang).
Debug mode is enabled for libstdc++ only.

libc++'s hardened mode is also enabled.

libstdc++'s debug mode has already helped find a few bugs:
- GLenumStrings.cpp: sRGB PVRTC v1 was in the wrong place.
- AmiiboData.cpp: Searching for character variants was incorrect.

Not enabled for Release builds, since the assertions cause the program
to crash with SIGABRT.

cmake/platform/gcc.cmake

@@ -284,6 +284,11 @@ SET(RP_CXX_FLAGS_RELEASE	"-O2 -DNDEBUG ${CFLAGS_VECTORIZE}")
 SET(RP_C_FLAGS_RELWITHDEBINFO	"-O2 -ggdb -DNDEBUG ${CFLAGS_VECTORIZE}")
 SET(RP_CXX_FLAGS_RELWITHDEBINFO	"-O2 -ggdb -DNDEBUG ${CFLAGS_VECTORIZE}")
 
+# Enable C++ assertions. (libstdc++ / libc++)
+# Also enable C++ debug mode. (libstdc++ only)
+# TODO: Only enable GLIBCXX for libstdc++, and LIBCPP for libc++.
+SET(RP_CXX_FLAGS_DEBUG "${RP_CXX_FLAGS_DEBUG} -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC -D_LIBCPP_ENABLE_ASSERTIONS -D_LIBCPP_ENABLE_HARDENED_MODE")
+
 # Unset temporary variables.
 UNSET(CFLAG_OPTIMIZE_DEBUG)