Fixed default value for CMAKE_INSTALL_PREFIX
Fixed default value for CMAKE_INSTALL_PREFIX

Stupid me: need to check if CMAKE_INSTALL_PREFIX is set by the user
before setting a default value. This is fixed now.

chrisemblhh/nexus,chrisemblhh/nexus,nexusformat/code,nexusformat/code,chrisemblhh/nexus,chrisemblhh/nexus,nexusformat/code,chrisemblhh/nexus,nexusformat/code,nexusformat/code