Add -Wswitch-enum to default GCC/Clang build options

This is now consistent with the warnings that Visual Studio would generate,
which warns about missing enum cases in switch/case even when a default: is
present.

Author: tautschnig

src/config.inc

@@ -5,7 +5,7 @@ BUILD_ENV = AUTO
 ifeq ($(BUILD_ENV),MSVC)
   #CXXFLAGS += /Wall /WX
 else
-  CXXFLAGS += -Wall -pedantic -Werror -Wno-deprecated-declarations
+  CXXFLAGS += -Wall -pedantic -Werror -Wno-deprecated-declarations -Wswitch-enum
 endif
 
 # Select optimisation or debug info