In make/sun/javac/javac/Makefile in ladybird workspace:

==========================
# Use bootstrap compiler.
# on linux we don't have one to boot from so cobble one together
ifeq ($(PLATFORM), linux)
JAVAC_CMD = $(BINDIR)/java -cp
$(BUILDDIR)/../share/tools/javac/gjc.jar:$(BUILDDIR)/../share/tools/javac/gjcmai
n.jar:$(BUILDDIR)/../share/tools/javac/jstyle.jar:$(BUILDDIR)/../share/tools/jav
ac/munge.jar com.sun.tools.javac.Main
else
JAVAC_CMD = $(JAVAC_BOOT_CMD)
endif
==========================

Apparently, the above four .jar files

        tools/javac/gjc.jar
        tools/javac/gjcmain.jar
        tools/javac/jstyle.jar
        tools/javac/munge.jar

are required for building linux j2se

Yet, they aren't included into the linux licensee source bundle, but
rather they are deposited into the plugin-all source bundle, making
the build of the j2se source bundle dependant on the plugin-all source
bundle.

According to Calvin Austin, as we already have 1.3 javac compiler installed on the linux build machine, ifeq ($(PLATFORM), linux) portion of this file is not needed anymore.