build: don't link everything to libcap

The intent of the assignments around the check for libcap was obviously
to avoid polluting LIBS. To work properly, LIBS must be restored from
the save_LIBS variable.

The practical effect is small though, because pretty much everything
links with libbasic, which links with CAP_LIBS.

configure.ac

@@ -293,6 +293,7 @@ save_LIBS="$LIBS"
 LIBS=
 AC_SEARCH_LIBS([cap_init], [cap], [], [AC_MSG_ERROR([*** POSIX caps library not found])])
 CAP_LIBS="$LIBS"
+LIBS="$save_LIBS"
 AC_SUBST(CAP_LIBS)
 
 AC_CHECK_FUNCS([memfd_create])