Fix a couple of compiler warnings

Admittedly, we shut up a few instead of fixing them.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
CheckSezpoz: Use the ToolProvider to get the 'apt' class

This is more robust than expecting the 'apt' main class to be available
in the default class path.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
CheckSezpoz: use ToolProvider.getSystemJavaCompiler()

... but fall back to calling the apt class directly if we're not running inside a JDK.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
CheckSezpozTest: use ToolProvider.getSystemJavaCompiler()

... instead of calling 'javac' directly.

This is more robust than expecting 'javac' to be in the PATH as long as
we are assuming that the regression tests are run within a JDK -- which
is usually the case because they are run in the context of building
scijava-common.

This fixes http://trac.imagej.net/ticket/1560.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>