Support parameter "gnuConfigurePrepareSkip".

Some source tree uses symbolic link.
But previous versions of Java doesn't API to create symlinks.
So it may causes strange behavior on the build stage. (refs #82)

GNU configure can build binaries on the different place from its source directory.
[like: "mkdir bin; cd bin; ../src/configure" ]
So in some cases, we don't need to copy.
But some source tree may have no configure but configure.ac.
In such cases, we should copy sources into target/gnu/** .

This patch provides parameter named "gnuConfigurePrepareSkip".
It has boolean value.
True means "not copy sources, not run autogen/buildconf."
The default is false. So it keeps the backward compatiblity.

gnuConfigurePrepareSkip has a good side-effect.
You can reduce your disk usage.
It may be remarkable if you build huge system like GCC.
Add postBuildHookScript element to the configuration of integrated-test.
Support parameter getGnuConfigureInstallPrefix.

Some GNU tools embed the value of "--prefix" you set to GNU configure.
The famous example is GCC. GCC searches its files from embedded prefix value.

It is provided DESTDIR makefile option in recent versions of GNU autoconf.
(Even if they might have no DESTDIR in ancient versions.)
You can install built binaries where you want to with DESTDIR.

This patch provides parameter named "getGnuConfigureInstallPrefix".
The type is strings.
When you specify it, GNU configure is called with "--prefix={your specidied path}".
GNU make is called with "DESTDIR="{install location specified by NAR plugin}".

The default value is empty.
When you don't specify it, NAR plugin works same as previous versions.
Add integrated tests.
Merge branch 'pr-add-postbuild-script-support' into work
Merge branch 'pr-support-gnuConfigurePrepareSkip' into work

Conflicts:
	src/main/java/com/github/maven_nar/NarGnuConfigureMojo.java
Merge branch 'pr-support-gnuConfigureInstallPrefix' into work

Conflicts:
	src/main/java/com/github/maven_nar/NarGnuConfigureMojo.java
	src/main/java/com/github/maven_nar/NarGnuMakeMojo.java