waf: prevent us from modifying non-install libs during make install

waf: prevent us from modifying non-install libs during make install

we need to ensure that 'make install' does not change any of our build
libraries, and only changes the .inst.so libraries, otherwise doing a
make test in the build directory directly after a make install could
use the installed libraries, which would mean using the wrong
LDB_MODULES_PATH

this could cause the "unknown error" loading ldb modules when running
some commands directly after a make install

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>