configury changes to make ld plugin support controlled by --enable-plugins

configury changes to make ld plugin support controlled by --enable-plugins

This also makes --enable-plugins default to on for hosts that can
support plugins, so we have consistent lto toolchain support.  The
ACX_LARGEFILE moves aren't strictly necessary, but are harmless and
will be necessary if plugin support is extended to more hosts via
libtool's dlopen support.  I started down that path then decided it
was more work than I was interested in doing.  (ACX_LARGEFILE invokes
AC_PLUGINS.)

config/
	* plugins.m4: Test for dlfcn.h or windows.h here to set default
	for --enable-plugins.  Report error if someone tries to enable
	plugins on a host we don't support.
bfd/
	* configure.ac: Delete redundant plugin related checks.
	* configure: Regenerate.
binutils/
	* configure.ac: Move ACX_LARGEFILE after LT_INIT.
	* config.in: Regenerate.
	* configure: Regenerate.
gas/
	* configure.ac: Move ACX_LARGEFILE after LT_INIT.
	* config.in: Regenerate.
	* configure: Regenerate.
gprof/
	* configure.ac: Move ACX_LARGEFILE after LT_INIT.
	* configure: Regenerate.
	* gconfig.in: Regenerate.
ld/
	* configure.ac: Move AC_PROG_CC and other macros earlier.  Delete
	plugin checks now done in config/plugins.m4.
	* config.in: Regenerate.
	* configure: Regenerate.