Allow the usage of non-standard JDKs for builds

java_home works for "standard" JDKs, but it is often desirable to have custom OpenJDK builds in
locations such as /opt.

This patch allows picking a correct $ALT_BOOTDIR when $JAVA_HOME is set, hence allowing building
OpenJDK with custom OpenJDK builds.
Merge pull request #4 from jponge/master

Seems good, merged
no more man found in jre/jdk produced dirs
no demo or sample in lambda
no src.zip in lambda
add XCLEAN support to lambda for linux
jre got man, whereas jdk don't have it...
demo, sample and src are back
hack version number only if version.numbers file exist first
pass MILESTONE at configure time
pass build number at configure time
get lambda build from jdk8 and now more lambda tag
man pages are back in Lambda
man pages are back in Lambda
no milestone or build-number configure options in Jigsaw yet
mention no more support for OSX
ensure bash is used under Linux (Ubuntu/Mint are dash based)
Sync Lambda and Jigsaw Linux build.sh
More Sync for Lambda and Jigsaw Linux build.sh
update packages so we could use FPM to generate DEB and RPMs