[patch] Remove NO_IMPLICIT_EXTERN_C target macro

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

Hello,

For targets that do not define NO_IMPLICIT_EXTERN_C, GCC assumes that
included system headers are implicitly wrapped in 'extern "C" {...}'.
I could not find where this target macro was introduced, but it is
very old (it is already mentioned in FSFChangeLog.10 for a change in
1995). AFAICT, today all supported targets have C++-compatible headers
(according to one comment, even hpux8 had C++-compatible system
headers :-), so NO_IMPLICIT_EXTERN_C shouldn't be necessary anymore.

The only target that explicitly undefines NO_IMPLICIT_EXTERN_C is
sparc64-openbsd, and I find it hard to believe that the system headers
on openbsd are not also already C++-compatible. Marc, could you have a
look at this, please?

The attached patch removes NO_IMPLICIT_EXTERN_C, and replaces its sole
user with IMPLICIT_EXTERN_C to avoid the double negations (#ifndef
NO_IMPLICIT_EXTERN_C, etc.).

Bootstrapped and tested on x86_64-unknown-linux-gnu. OK for trunk?

Ciao!
Steven

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