CMakeLists: increase strict aliasing level to 3

The strict aliasing rules disallow dereferencing the pointer to a
variable of a certain type as another type, which is frequently used
e.g. when casting structs to their base type. We currently have the
warning level for strict aliasing rules set to `2`, which is described
by gcc(1) as being "Aggressive, quick, not too precise." And in fact, we
experience quite a lot of warnings when doing a release build due to
that.

GCC provides multiple levels, where higher levels are more accurate, but
also slower due to the additional analysis required. Still, we want to
have warning level 3 instead of 2 to avoid the current warnings we have
in the Travis CI release builds. As this is the default warning level
when no level is passed to `-Wstrict-aliasing`, we can just remove the
level and use that default.

CMakeLists.txt

@@ -218,7 +218,7 @@ ELSE ()
 
 	ENABLE_WARNINGS(documentation)
 	DISABLE_WARNINGS(missing-field-initializers)
-	ENABLE_WARNINGS(strict-aliasing=2)
+	ENABLE_WARNINGS(strict-aliasing)
 	ENABLE_WARNINGS(strict-prototypes)
 	ENABLE_WARNINGS(declaration-after-statement)
 	DISABLE_WARNINGS(unused-const-variable)