[Bug preprocessor/85211] -isystem automatically adds extern "C" to includes

[blastrock at free dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85211

--- Comment #5 from blastrock at free dot fr ---
I tried using their recipe to rebuild gcc. The configure line is the following:

    $ /media/usb1/vitasdkbuild/build/gcc-final-prefix/src/gcc-final/configure
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=arm-vita-eabi
--prefix=/media/usb1/vitasdkbuild/build/vitasdk
--libdir=/media/usb1/vitasdkbuild/build/vitasdk/lib
--libexecdir=/media/usb1/vitasdkbuild/build/vitasdk/lib
--with-sysroot=/media/usb1/vitasdkbuild/build/vitasdk/arm-vita-eabi
--with-gmp=/media/usb1/vitasdkbuild/build/deps_build
--with-mpfr=/media/usb1/vitasdkbuild/build/deps_build
--with-mpc=/media/usb1/vitasdkbuild/build/deps_build
--with-isl=/media/usb1/vitasdkbuild/build/deps_build
--with-libelf=/media/usb1/vitasdkbuild/build/deps_build
--with-python-dir=share/gcc-arm-vita-eabi --enable-languages=c,c++
--disable-decimal-float --disable-libffi --disable-libgomp --disable-libmudflap
--disable-libquadmath --disable-libssp --disable-libstdcxx-pch --disable-nls
--disable-shared --disable-tls --with-gnu-as --with-gnu-ld --with-newlib
--disable-multilib --with-arch=armv7-a --with-tune=cortex-a9 --with-fpu=neon
--with-float=hard --with-mode=thumb --with-pkgversion=GNU Tools for ARM
Embedded Processors --with-host-libstdcxx=-static-libgcc
-Wl,-Bstatic,-lstdc++,-Bdynamic -lm --with-headers=yes --enable-threads=posix

You can see --with-newlib in there. However I can confirm that
NO_IMPLICIT_EXTERN_C is not defined as I tried inserting broken code in the
#ifndef NO_IMPLICIT_EXTERN_C in parser.c and it did break compilation.

So is this a regression since #37727 was fixed?