Please enhance the implementation of <javac> to run the compiler program named 
by the Ant property ${build.compiler.executable} if <javac> has no executable 
attribute.

This would enable adjacent definitions of the compiler and its version.  For 
example (in a .properties file):

build.compiler=jikes

build.compiler.executable=T:/tools/jikes/1.20/bin/jikes

This would be better than setting the PATH environment variable so that `jikes` 
resolves to the desired version, in two ways: it eases maintenance, by placing 
related parameters together; and it's less apt to have undesired side-effects 
such as causing child scripts or <ant> invocations to run a different compiler.

This could be implemented in Javac.getExecutable, but perhaps that's a bad idea.