Add flags needed for Clang to the build system.

This uses a variable CLANG that should be set to the Clang version. For
now, this has to be done manually (e.g. when invoking jam using jam
-sCLANG=34), but later, this will be auto-detected.

Work towards bug #10396

build/jam/ArchitectureRules

@@ -28,7 +28,10 @@ rule ArchitectureSetup architecture
 	#		our gcc4 compiler. See this discussion on some issues:
 	# http://www.freelists.org/post/haiku-development/hrev45320-Yet-another-nonobvious-effect-of-ftreevrp-optimization
 	if $(gccVersion[1]) >= 3 {
-		gccBaseFlags += -fno-strict-aliasing -fno-tree-vrp ;
+		gccBaseFlags += -fno-strict-aliasing ;
+		if ! $(CLANG) {
+			gccBaseFlags += -fno-tree-vrp ;
+		}
 	}
 
 	# disable array bounds warnings on gcc 4.6 or newer since they trigger
@@ -473,8 +476,12 @@ rule ArchitectureSetupWarnings architecture
 	HAIKU_WERROR_ARCH = $(architecture) ;
 
 	rule EnableWerror dirTokens : scope {
-		SetConfigVar WARNINGS : HAIKU_TOP $(dirTokens) : treatAsErrors
-			: $(scope) ;
+		# Clang gives way more warnings than GCC, so that code won't compile
+		# -Werror when using Clang.
+		if ! $(CLANG) {
+			SetConfigVar WARNINGS : HAIKU_TOP $(dirTokens) : treatAsErrors
+				: $(scope) ;
+		}
 	}
 
 	# Work-around for GCC 2 problem -- despite -Wno-multichar it reports
@@ -486,6 +493,15 @@ rule ArchitectureSetupWarnings architecture
 		WARNINGS on $(file) = $(WARNINGS) ;
 	}
 
+	if $(CLANG) {
+		# We need -integrated-as, as otherwise Clang uses GCC as assembler and
+		# passes -fheinous-gnu-extensions to GCC, which GCC does not understand
+		# then errors out.
+		AppendToConfigVar CCFLAGS :
+			HAIKU_TOP src system libroot posix glibc :
+			-integrated-as -fgnu89-inline -fheinous-gnu-extensions : global ;
+	}
+
 	EnableWerror src add-ons accelerants 3dfx ;
 	EnableWerror src add-ons accelerants ati ;
 	EnableWerror src add-ons accelerants common ;