Add user settable HAIKU_REVISION build variable

Can be set to a revision string (hrev4...) to override the revision
determined via git. Useful when the repository doesn't have tags.

build/jam/FileRules

@@ -375,10 +375,12 @@ rule DetermineHaikuRevision
 	local gitIndex = <haiku-rootdir-git>index ;
 	local revisionFile = <haiku-rootdir-git>haiku-revision ;
 	if ! [ on $(gitIndex) return $(HAIKU_GIT_REVISION_DETERMINED) ] {
-		if [ Glob [ FDirName $(HAIKU_TOP) .git ] : index ] {
+		MakeLocate $(revisionFile) : $(HAIKU_BUILD_OUTPUT_DIR) ;
+		if $(HAIKU_REVISION) {
+			DetermineHaikuRevision2 $(revisionFile) ;
+		} else if [ Glob [ FDirName $(HAIKU_TOP) .git ] : index ] {
 			HAIKU_GIT_REVISION_DETERMINED on $(gitIndex) = 1 ;
 			SEARCH on $(gitIndex) = [ FDirName $(HAIKU_TOP) .git ] ;
-			MakeLocate $(revisionFile) : $(HAIKU_BUILD_OUTPUT_DIR) ;
 			Depends $(revisionFile) : $(gitIndex) ;
 			DetermineHaikuRevision1 $(revisionFile) : $(gitIndex) ;
 		} else {
@@ -396,6 +398,12 @@ actions DetermineHaikuRevision1
 }
 
 
+actions DetermineHaikuRevision2
+{
+	echo $(HAIKU_REVISION) > $(1)
+}
+
+
 rule DataFileToSourceFile sourceFile : dataFile : dataVariable : sizeVariable
 {
 	sourceFile = [ FGristFiles $(sourceFile) ] ;

build/jam/UserBuildConfig.ReadMe

@@ -87,6 +87,10 @@ HAIKU_DONT_CLEAR_IMAGE = 1 ;
 # several times larger than even with level 1.
 HAIKU_PACKAGE_COMPRESSION_LEVEL = 1 ;
 
+# Override the Haiku revision string determined via git. This is e.g. useful,
+# when your git repository doesn't have hrev tags. You may need to remove the
+# file <generated>/build/haiku-revision whenever you change this variable.
+HAIKU_REVISION = hrev66666 ;
 
 # Affects the haiku-image, haiku-vmware-image, and install-haiku targets. Only
 # targets on which the HAIKU_INCLUDE_IN_IMAGE variable has been set will be