RFR: JDK-8136695 Automatic build comparison with COMPARE_BUILD

[Magnus Ihse Bursie]

A common scenario in when working in the build system is to want to 
check if a particular change results in a change in the build output or 
not, or a more detailed analysis of such changes.

The compare.sh tool will provide such an analysis, but only if you have 
a pre-built baseline to compare with.

To facilitate testing changes, especially on distributed job servers, we 
should add a COMPARE_BUILD make control variable. If this variable is 
present, the build will run twice, the second time with the code that 
should be tested (particular make targets, configure arguments, or with 
a patch file applied), and then automatically run the compare script 
afterwards against the baseline.

The suggested format allows for a very fine-grained control, by further 
parsing the value of COMPARE_BUILD into separate parts. The syntax looks 
like this:
COMPARE_BUILD=CONF=<configure options>:PATCH=<patch file>:MAKE=<make 
targets>:COMP_OPTS=<compare script options>|<default>

If neither CONF or PATCH is given, assume <default> means CONF if it 
begins with "--", otherwise assume it means PATCH. Either CONF or PATCH 
must be given, directly specified or by using the default syntax. The 
rest of the key/value pairs are optional, and they may come in any order.

MAKE and COMP_OPTS can only be used with CONF and/or PATCH specified. If 
any value contains "+", it will be replaced by space.
If just a single value is specified, it can contain spaces, e.g. 
"COMPARE_BUILD=CONF=--enable-debug --with-special-sauce". (For technical 
reasons, this is not possible if using multiple, colon-separated 
key=value pairs).

Values given in CONF and MAKE are appended to the normal configure 
command line and make target list, respectively, when building the 
second comparison round.

The patch file, if specified, could be either absolute or relative to 
TOPDIR.

COMP_OPTS is additional options to pass to compare.sh.

Bug: https://bugs.openjdk.java.net/browse/JDK-8136695
WebRev: http://cr.openjdk.java.net/~ihse/JDK-8136695-compare_build/webrev.01

/Magnus