If we're not making javac run in a subprocess, libraries the project
depends on which leiningen has another version of will clash. This is
because leiningen append its standalone to java's bootclasspath, which
makes the ToolProvider add these classpaths automatically into the java
compiler it returns. By starting a subprocess without leiningen added to
the bootclasspath, we avoid this library clashing.
UI changes as a result of this patch: Whenever javac fails, the task one
wanted to run will be responsible for aborting Leiningen, whereas javac
did this itself when it discovered that the compilation failed. As we're
running in a subprocess, an abort message such as
Uberjar aborting because jar/compilation failed: Subprocess failed
will appear, though this will only be appended to the javac error
message and the "Compilation of Java sources (lein javac) failed."
This commit also adds the overhead of starting a subprocess when one
have to compile java sources, but it will not start one if no
.class-files are outdated/not existing.
Not tested on Windows.