package kit: Disable curl requirement on bootstrap build

* As per the mailing list.
* Introduce HAIKU_BOOTSTRAP_BUILD define to sources
  to let them know they are taking part in a bootstrap

build/jam/DefaultBuildProfiles

@@ -2,6 +2,8 @@
 switch $(HAIKU_BUILD_PROFILE) {
 	case "bootstrap-*" :
 		HAIKU_BOOTSTRAP_BUILD = 1 ;
+		HAIKU_DEFINES += HAIKU_BOOTSTRAP_BUILD ;
+		TARGET_DEFINES += HAIKU_BOOTSTRAP_BUILD ;
 }