loader: Allow null string for CMAKE_INSTALL_PREFIX to prevent extra paths

NULL string was making loader search extra local paths such as /share/...
Also expand log buffer .
diff --git a/CMakeLists.txt b/CMakeLists.txt
index fd594c2..9a21f0e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -43,6 +43,7 @@
     add_definitions(-DSYSCONFDIR="${CMAKE_INSTALL_SYSCONFDIR}")
     add_definitions(-DDATADIR="${CMAKE_INSTALL_DATADIR}")
     if (CMAKE_INSTALL_PREFIX STREQUAL "/usr")
+    elseif (CMAKE_INSTALL_PREFIX STREQUAL "")
     else()
         add_definitions(-DLOCALPREFIX="${CMAKE_INSTALL_PREFIX}")
     endif()