diff --git a/src/add-ons/kernel/drivers/network/rtl8139/Jamfile b/src/add-ons/kernel/drivers/network/rtl8139/Jamfile
index fc58b43450..8c62ca0e66 100644
--- a/src/add-ons/kernel/drivers/network/rtl8139/Jamfile
+++ b/src/add-ons/kernel/drivers/network/rtl8139/Jamfile
@@ -1,16 +1,7 @@
 SubDir OBOS_TOP src add-ons kernel drivers network rtl8139 ;
 
-# set some additional defines
-{
-	local defines ;
-
-	if $(DEBUG) {
-		defines += DEBUG ;
-	}
-
-	defines = [ FDefines $(defines) ] ;
-	SubDirCcFlags $(defines) -Wall -Wno-multichar ;
-}
+# set some additional flags
+SubDirCcFlags -Wall ;
 
 R5KernelAddon rtl8139 : [ FDirName kernel drivers network ] :
 	driver.c
diff --git a/src/add-ons/kernel/drivers/network/sis900/Jamfile b/src/add-ons/kernel/drivers/network/sis900/Jamfile
index c5ecc960d0..179ec881ce 100644
--- a/src/add-ons/kernel/drivers/network/sis900/Jamfile
+++ b/src/add-ons/kernel/drivers/network/sis900/Jamfile
@@ -1,16 +1,7 @@
 SubDir OBOS_TOP src add-ons kernel drivers network sis900 ;
 
-# set some additional defines
-{
-	local defines ;
-
-	if $(DEBUG) {
-		defines += DEBUG ;
-	}
-
-	defines = [ FDefines $(defines) ] ;
-	SubDirCcFlags $(defines) -Wall -Wno-multichar ;
-}
+# set some additional flags
+SubDirCcFlags -Wall ;
 
 R5KernelAddon sis900 : [ FDirName kernel drivers network ] :
 	driver.c
diff --git a/src/add-ons/kernel/file_systems/bfs/Jamfile b/src/add-ons/kernel/file_systems/bfs/Jamfile
index 5ebf92c937..ca67a6cbf3 100644
--- a/src/add-ons/kernel/file_systems/bfs/Jamfile
+++ b/src/add-ons/kernel/file_systems/bfs/Jamfile
@@ -13,9 +13,7 @@ oldOPTIM = $(OPTIM) ;
 		defines += COMPILE_FOR_R5 ;
 	}
 
-	if $(DEBUG) {
-		defines += DEBUG ;
-	} else {
+	if $(DEBUG) = 0 {
 		# the gcc on BeOS doesn't compile BFS correctly with -O2 or more
 		OPTIM = -O1 ;
 	}
diff --git a/src/add-ons/kernel/file_systems/iso9660/Jamfile b/src/add-ons/kernel/file_systems/iso9660/Jamfile
index 1a50a988bf..5fc4b0fe8b 100644
--- a/src/add-ons/kernel/file_systems/iso9660/Jamfile
+++ b/src/add-ons/kernel/file_systems/iso9660/Jamfile
@@ -13,9 +13,7 @@ oldOPTIM = $(OPTIM) ;
 		defines += COMPILE_FOR_R5 ;
 	}
 
-	if $(DEBUG) {
-		defines += DEBUG ;
-	} else {
+	if $(DEBUG) = 0 {
 		# the gcc on BeOS doesn't compile BFS correctly with -O2 or more
 		OPTIM = -O1 ;
 	}
diff --git a/src/add-ons/kernel/file_systems/udf/Jamfile b/src/add-ons/kernel/file_systems/udf/Jamfile
index 3ddd643f81..501fd167f5 100644
--- a/src/add-ons/kernel/file_systems/udf/Jamfile
+++ b/src/add-ons/kernel/file_systems/udf/Jamfile
@@ -13,9 +13,7 @@ oldOPTIM = $(OPTIM) ;
 		defines += COMPILE_FOR_R5 ;
 	}
 
-	if $(DEBUG) {
-		#defines += DEBUG ;
-	} else {
+	if $(DEBUG) = 0 {
 		# the gcc on BeOS doesn't compile BFS correctly with -O2 or more
 		OPTIM = -O1 ;
 	}