Improve Linux Makefile.

The following changes have been implemented:

  * make sure all standard (eg. $CFLAGS) variables are taken into
    account when compiling;
  * define default compilation flags in separate $MAINT_* variables
    so that user-defined variables can override them;
  * split default flags the expected way, eg. $CPPFLAGS instead of
    $CFLAGS for C preprocessor flags;
  * declare all dependencies, including the ones on header files;
  * install localized man pages in the corresponding locale-qualified
    directories, so that man(1) can pick them up automatically;
  * install .desktop file;
  * support $DESTDIR for downstream maintainers' convenience;
  * provide uninstall target;
  * get rid of symlink hackery.