add --with-install-dir= configure option
add install-src target

It creates the source code (`allsrc' target) if ENABLE_META == yes.
Afterwards it will create the install directory.
keep relative path of installation dir

This gives more freedom to the user in case the FlexibleSUSY direcory
is moved.
implement install-src target in the src/ module
use install instead of cp, and install -d instead of mkdir -p
checking for avaliability of `install'
print error message if installation directory is not set
do not include dependency files for install-src target
install config, legacy, slhaea and the model files
set access rights the same as source
correcting access righs to u=rw,g=r,o=r
install doc module
invoke error only if install-src target is called
add script that removes marked lines from file or stdin
improve readability in the error message
do not create standalone makefile in the exported version
export doc/module.mk
adding helper script which installs a file, w/o export markers
installing stripped versions of configure and Makefile.in
export generated makefile modules two_scale_*.mk