configure: don't give an example including an equals sign

We can't handle --prefix=foo (we need --prefix foo instead).
So don't give the former as an example.

Signed-off-by: Colin McCabe <cmccabe@alumni.cmu.edu>
CMakeLists.txt: honor CMAKE_INSTALL_PREFIX if it's already set

Signed-off-by: Colin McCabe <cmccabe@alumni.cmu.edu>