Install Prefix suggestion

[Pickle]

I was cross compiling the library and wanted it to install to the toolchain. The hardcoded paths didnt work. Also not sure why the built lib is being copied to its own folder beneath the lib folder. I came up with this change and it worked. Maybe something like this is proper for all systems?

     install(TARGETS GL
       LIBRARY
-      DESTINATION "/usr/lib/gl4es/"
+      DESTINATION "${CMAKE_INSTALL_PREFIX}/lib"
     )
     install(FILES "../include/gl4esinit.h" "../include/gl4eshint.h"
-      DESTINATION "/usr/include/gl4es/"
+      DESTINATION "${CMAKE_INSTALL_PREFIX}/include/gl4es/"
     )