Error on Gentoo

I am stuck at compiling this on Gentoo with g++ 8.3.0
After I typed

`make clean all
`

The following occoured:

`rm -f src/font.o src/glyph.o src/normalize.o src/table_tags.o src/transform.o src/woff2_dec.o src/woff2_enc.o src/woff2_common.o src/woff2_out.o src/variable_length.o src/woff2_compress.o src/woff2_decompress.o src/woff2_info.o woff2_compress woff2_decompress woff2_info
make -C brotli clean
make[1]: Entering directory '/home/null/woff2/brotli'
rm -rf bin libbrotli.a
make[1]: Leaving directory '/home/null/woff2/brotli'
c++ -I./brotli/c/include/ -I./src -I./include -c -o src/font.o src/font.cc
c++: error trying to exec 'cc1plus': execvp: Not a directory
make: *** [: src/font.o] Error 1
`

But I have found cc1plus in path:/usr/libexec/gcc/x86_64-pc-linux-gnu/8.3.0/
So I tried to

`export PATH=/usr/libexec/gcc/x86_64-pc-linux-gnu/8.3.0/:$PATH
`

and then

`make clean all
`

but the error happen again.
What should I do to fix this?