build: Unify BeOS ABI compatibility ifdefs around __HAIKU_BEOS_COMPATIBLE.

_BEOS_R5_COMPATIBLE_ was defined in ArchitectureRules while
__HAIKU_BEOS_COMPATIBLE is defined in HaikuConfig.h (which is
in the include path for sys/types, SupportDefs, and other
base headers.)

build/jam/ArchitectureRules

@@ -131,6 +131,8 @@ rule ArchitectureSetup architecture
 		# But these can stay.
 		HAIKU_WERROR_FLAGS_$(architecture) += -Wno-error=cast-align
 			-Wno-error=format-truncation ;
+	} else {
+		HAIKU_WERROR_FLAGS_$(architecture) += -Wno-unknown-pragmas ;
 	}
 
 	# debug flags
@@ -150,11 +152,6 @@ rule ArchitectureSetup architecture
 	# TODO: Temporary work-around. Should be defined in the compiler specs
 	HAIKU_LINKFLAGS_$(architecture) += -Xlinker --no-undefined ;
 
-	if $(HAIKU_CC_IS_LEGACY_GCC_$(architecture)) = 1 {
-		HAIKU_DEFINES_$(architecture) += _BEOS_R5_COMPATIBLE_ ;
-		HAIKU_WERROR_FLAGS_$(architecture) += -Wno-unknown-pragmas ;
-	}
-
 	# private shared kernel/libroot headers
 	HAIKU_PRIVATE_SYSTEM_HEADERS_$(architecture)
 		= [ PrivateHeaders $(DOT) system system/arch/$(cpu) ] ;