Install apache module into user's libexec dir

I tried php-build with "with_apxs2", then I got following errors.

```
/usr/share/httpd/build/instdso.sh SH_LIBTOOL='/usr/share/apr-1/build-1/libtool' libs/libphp5.so /usr/libexec/apache2
cp: /usr/libexec/apache2/libphp5.so: Permission denied
apxs:Error: Command failed with rc=65536
.
make: *** [install-sapi] Error 1
```

Of cource I can install apache module by typing "sudo php-build", however i think it's not expected behavior.

Not only PHP binaries but also Apache module should be installed to user's directory such as "5.4.13/libexec/apache2/libphp5.so". This PR changes libexec path to user's directory.