Error on Gentoo

[C-Entropy]

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: *** [<builtin>: 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?