with current tip 7126:503de53783a4

if I use "./waf --enable-modules" outside of configure, it results in an error

./waf distclean
./waf --enable-modules=lte
Checking for program gcc or cc           : /usr/bin/gcc 
Checking for program cpp                 : /usr/bin/cpp 
[snip]
Waf: Entering directory `/home/nicola/locale/ns-3-dev/build'
the option --enable-modules is being applied to this build only; to make it permanent it needs to be given to waf configure.
Waf: Leaving directory `/home/nicola/locale/ns-3-dev/build'
object 'ns3-internet' was not found in uselib_local (required by 'ns3-lte')


note that if I do:
./waf distclean
./waf configure --enable-modules=lte
./waf

then it works properly.

The problem is that it currently seems that this option is supported. If we don't support it then waf should report a meaningful error.

I think this