build-sys: Update configure.ac

build-sys: Update configure.ac

1. the test incorrectly used AC_COMPILE_IFELSE instead of
   AC_LINK_IFELSE, defeating the purpose of checking -lcrypt.
2. the test did not properly restore LIBS, causing later checks to all
   fail if libcrypt wasn't found.
3. HAVE_LIBCRYPT only controls whether to use -lcrypt, it is not
   needed or used in any source files.

[kzak@redhat.com: - improve commit message
                  - use UL_{SET,RESTORE}_FLAGS() rather than directly
		    modify $LIBS]

Signed-off-by: Karel Zak <kzak@redhat.com>