configure: Further fixes to Clang support.

* Actually locate the clang executable, and allow user overrides
 * Properly preserve arguments in get_build_tool_path
 * Fix get_build_tool_path for commands with dashes (e.g. "clang-5.0")

build/jam/ArchitectureRules

@@ -34,6 +34,10 @@ rule ArchitectureSetup architecture
 		}
 	}
 
+	if $(HAIKU_CC_IS_CLANG_$(architecture)) = 1 {
+		gccBaseFlags += -fno-builtin-vfork ;
+	}
+
 	# disable array bounds warnings on gcc 4.6 or newer since they trigger
 	# too many false positives. Coverity does a better job of this kind of
 	# analysis anyways.
@@ -132,6 +136,10 @@ rule ArchitectureSetup architecture
 		HAIKU_WERROR_FLAGS_$(architecture) += -Wno-unused-but-set-variable ;
 	}
 
+	if $(HAIKU_CC_IS_CLANG_$(architecture)) = 1 {
+		gccBaseFlags += -Wno-address-of-packed-member -Wno-unused-private-field ;
+	}
+
 	# debug flags
 	local debugFlags = -ggdb ;