Someone

# Prevent the configure script from continuing any further if
# configuration is being performed in the top-level directory.
# The idea is to prevent this ,because some maintainers tend
# to break parallel build trees (a.k.a. VPATH builds).
if test "$srcdir" = "." && test "$enable_maintainer_mode" != "yes"; then
  AC_MSG_ERROR(
     [
      Please configure and build in a directory other than the
      top-level source directory. This is needed because a lot
      of maintainers tend to break parallel build trees
      (a.k.a. VPATH builds). This is kinda real ensurance they
      will not do it (by enforcing everybody to do VPATH builds).

      For example, try the following from the top-level source
      directory:

          mkdir objdir
          cd objdir
          ../configure
          make

      This will create a build space in the directory `objdir' and
      start a build in that directory.

      If however you realy want to disable this error,
      use --enable-maintainer-mode switch.
     ])
fi