py38-pyqt5 @5.14.1 build fails: cannot find sip.h (provided by py38-sip)

[snowflake (Dave Evans)]

The build works on some versions of macOS, but not on mine which is 10.11

The build cannot find header file sip.h because the correct include directory is not specified.

sip.h is here

/opt/local/Library/Frameworks/Python.framework/Versions/3.8/include/python3.8m/sip.h

Here is the failing command

/Library/Developer/CommandLineTools/usr/bin/clang++ -c -pipe -stdlib=libc++ -Os -std=gnu++11  -arch x86_64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -mmacosx-version-min=10.11 -fno-exceptions -Wall -W -fPIC -DSIP_PROTECTED_IS_PUBLIC -Dprotected=public -DPy_LIMITED_API=0x03040000 -DQT_NO_EXCEPTIONS -DQT_NO_DEBUG -DQT_PLUGIN -DQT_CORE_LIB -I. -I. -I/opt/local/Library/Frameworks/Python.framework/Versions/3.8/include/python3.8 -I../qpy/QtCore -I/opt/local/libexec/qt5/lib/QtCore.framework/Headers -I. -I/opt/local/libexec/qt5/mkspecs/macx-clang -F/opt/local/libexec/qt5/lib -o sipQtCoreQAbstractAnimation.o sipQtCoreQAbstractAnimation.cpp
In file included from sipQtCoreQAbstractAnimation.cpp:24:
./sipAPIQtCore.h:31:10: fatal error: 'sip.h' file not found
#include <sip.h>
         ^
In file included from qpycore_post_init.cpp:25:
In file included from ../qpy/QtCore/qpycore_api.h:30:
../qpy/QtCore/qpycore_public_api.h:26:10: fatal error: 'sip.h' file not found
#include <sip.h>
         ^
1 error generated.

[snowflake (Dave Evans)]

This is a duplicate of #60114