Fix plugin path detection when installed with DESTDIR.

This is in no way perfect but should cover most common cases.

Closes #291

src/config-keepassx.h.cmake

@@ -8,8 +8,8 @@
 #define KEEPASSX_SOURCE_DIR "${CMAKE_SOURCE_DIR}"
 #define KEEPASSX_BINARY_DIR "${CMAKE_BINARY_DIR}"
 
+#define KEEPASSX_PREFIX_DIR "${CMAKE_INSTALL_PREFIX}"
 #define KEEPASSX_PLUGIN_DIR "${PLUGIN_INSTALL_DIR}"
-
 #define KEEPASSX_DATA_DIR "${DATA_INSTALL_DIR}"
 
 #cmakedefine HAVE_PR_SET_DUMPABLE 1