[v3] --enable-debug

[Benjamin Kosnik <bkoz at redhat dot com>]

This patch implements --enable-debug, --enable-debug-flags=FLAGS, and
cleans up various configure and make issues, including build-time
dependency issues*.

I did not do --enable-debug-libraries=FLAGS. I'm hoping somebody else will.

In a nutshell, default behavior has not changed.

When the --enable-debug configure option is passed, a parallel set of
libraries is built in src/debug, and installed in $(prefix)/lib/debug.
This debug library is by default built with '-g3 -O0': to change the
flags that this library is built with, use....
--enable-debug-flags=MY_SPECIAL_FLAGS.

tested x86/linux
tested x86/linux --enable-clocale=generic
tested x86/linux --enable-debug
tested x86/linux --enable-debug --enable-debug-flags="-gdwarf2"

testing in progress x86/linux x xscale-elf

* The following now fixed:
1) libstdc++-v3/Makefile 'multi-do' growing target overrides
2) include/Makefile target hicups
3) src dependencies for configure source files
4) ... leading to a failure of './config.status; cd src; make' to work
as expected. It now does.

Some of these have annoyed me for years.

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