Missing system headers while installing GCC 9.1.0 on macOS 10.15.5

I'm trying to build GCC from source and have managed to build all the dependencies, however right now I'm struggling with building GCC itself. In macOS 10.15 Apple deprecated the location /usr/include, and the build script is trying to locate the headers there, however the headers are located at /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include

The error:

The directory that should contain system headers does not exist:  /usr/includemake[3]: *** [stmp-fixinc] Error 1make[2]: *** [all-stage1-gcc] Error 2make[1]: *** [stage1-bubble] Error 2make: *** [all] Error 2

What I've tried:

1. https://developer.apple.com/forums/thread/104296 (outdated, in macOS 10.15 Apple doesn't provide package to install system headers in /usr/include)

2. configure options:

   • --with-native-system-header-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include (Trouble building gcc on Mac - can't find system headers)
   • --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include
   • --with-build-sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include
   • --with-sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79885)
   • --with-build-sysroot=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79885)

3. Changing the oldincludedir variable in the Makefile (generated by the configure script) from /usr/include to the correct path.

4. SYSTEM_HEADER_DIR=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/

5. CPATH=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include

6. Symlinking /usr/include with /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include is unfeasible.

Any suggestions are much appreciated!Thanks in advance!