Build gcc-9.1.0 error with libgfortran.so

Question

I tried to install gcc-9.1.0 version with gcc-5 and the following configure options:

../gcc-9.1.0/configure --prefix=local/gcc/9.1.0-all \
   --program-suffix='-10' \
   --without-included-gettext \
   --enable-threads=posix \
   --enable-libstdcxx-time=yes \
   --enable-shared \
   --enable-languages=c,c++,fortran,go,objc,obj-c++ \
   --with-mpfr="$MPFR_DIR" \
   --with-mpc=$MPC_DIR\
   --with-gmp=$GMP_DIR\
   --with-isl=$ISL_DIR\
   --enable-host-shared \
   --disable-libquadmath --disable-libquadmath-support --disable-werror\
   CC=gcc-5 CXX=g++-5

However, I always get the error:

../gcc-9.1.0/libgfortran/runtime/string.c:96:1: error: static declaration of ‘strnlen’ follows non-static declaration

How can I resolve this error? Can't I install gfortran under current system?

My installation system is Ubuntu 16.04.5 LTS (Xenial Xerus) and CPU is Intel(R) Xeon(R) CPU E3-1231 v3 @ 3.40GHz.

After install gcc-9 and g++-9, I try to install only gfortran as following configuration:

../gcc-9.1.0/configure --prefix=$EXTERNALPACKAGES/$PACKAGE/$VER-fortran \
   --program-suffix='-9' \
   --without-included-gettext \
   --enable-threads=posix \
   --enable-libstdcxx-time=yes \
   --with-system-zlib \
   --disable-browser-plugin \
   --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu \
   --enable-shared \
   --enable-languages=fortran \
   --with-mpfr="$MPFR_DIR" \
   --with-mpc=$MPC_DIR\
   --with-gmp=$GMP_DIR\
   --with-isl=$ISL_DIR\
   --enable-host-shared \
   --disable-multilib \
   --disable-libquadmath --disable-libquadmath-support --disable-werror\
   CC=gcc-9 CXX=g++-9

However, I got same error:

../../../gcc-9.1.0/libgfortran/runtime/string.c:96:1: error: static dec
laration of ‘strnlen’ follows non-static declaration