Compile GCC 7 : error: C++ preprocessor "/lib/cpp" fails sanity check

Trying to build GCC 7.1.0 from source. This is not my first time and it used to work in past GCC releases. ``` ../configure --build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 \ --prefix=/c/mingw64 --with-sysroot=/c/mingw64 \ --with-gmp=/c/mingw64/gmp --with-mpfr=/c/mingw64/mpfr --with-mpc=/c/mingw64/mpc \ --with-isl=/c/mingw64/isl \ --disable-nls --disable-multilib --disable-libstdcxx-pch --disable-shared \ --disable-win32-registry --disable-libstdcxx-debug --disable-libstdcxx-verbose \ --with-tune=haswell --enable-lto --enable-checking=release --enable-languages=c,c++ \ --enable-libstdcxx-time --enable-threads=win32 --enable-libatomic --enable-fully-dynamic-string ``` Error ``` checking how to run the C++ preprocessor... /lib/cpp configure: error: in `/c/mingw64/src/build/gcc': configure: error: C++ preprocessor "/lib/cpp" fails sanity check See `config.log' for more details. make[2]: *** [Makefile:4309: configure-stage2-gcc] Error 1 make[2]: Leaving directory '/c/mingw64/src/build' make[1]: *** [Makefile:20550: stage2-bubble] Error 2 make[1]: Leaving directory '/c/mingw64/src/build' make: *** [Makefile:936: all] Error 2 ``` Complete config.log: https://pastebin.com/raw/mEeJHCuK Note that I `/lib/cpp` never exists. I did some google search and tried every suggestions, including: - `CXX=/c/mingw/bin/g++` (likewise to `CC` and `CPP`) - Replace all occurances of `/lib/cpp` in `gcc-7.1.0/gcc/configure` with `/c/mingw/bin/g++`. Which gave me: ``` conftest.c:14:8: error: 'Syntax' does not name a type Syntax error ^ ``` (^ is similar to Compiling on a mac: What does it mean if my compiler fails a sanity check?) There was an old (critical) bug that seems related at https://gcc.gnu.org/ml/gcc-bugs/2015-10/msg00604.html , but no update.