If I make a change to a single build tool (say, adding a blank line into jdk/make/src/classes/build/tools/dtdbuilder/DTDBuilder.java) and rebuild, all other tools are also built and all gendata re-generated:

## Starting jdk
Compiling 151 files for BUILD_TOOLS
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
Compiling 390 properties into resource bundles
Creating sun/util/LocaleDataMetaInfo.java from 422 found resources.
Generating sun/misc/Version.java
Generating sun/misc/Version.java compact1
Generating sun/misc/Version.java compact2
Generating sun/misc/Version.java compact3
Generating beaninfo
Generating Nimbus source files
Generating HTML DTD file
Generating blacklisted certs
[Parsed DTD html32 in 56ms]
## Finished jdk (build time 00:00:35)

This is a waste of time.