Fix a regression in include paths when compiling ctypes in 3.6.15/3.7.12. (#2209)

In my previous work on getting Python 3.6.15 and 3.7.12 to compile on
Apple M1, I backported logic from newer 3.8.x releases to properly find
libffi and related files on macOS.

This regressed compilation on Linux. The include search path was
incomplete, and `ffi.h` could not be found, resulting in `ctypes` being
disabled.

There was a key difference between the old logic and new logic that led
to this regression:

1. In 3.8 and newer, `detect_ctypes()` in `setup.py` took no arguments,
   and was expected to access instance variables for the include search
   path.
2. In 3.7 and earlier, `detect_ctypes()` took the path as an argument,
   and was expected to make use of it.

The backport made use of the instance variables, overriding the provided
include path. These were not equivalent. The one on the instance was not
complete, lacking the necessary directories to find `ffi.h`. Since this
could not be found, `ctypes` support was disabled.

The fix is to simply not overwrite the variables passed to the function,
and resume using them as before.

Fixes #2207

plugins/python-build/share/python-build/patches/3.6.15/Python-3.6.15/0004-Port-ctypes-and-system-libffi-patches-for-arm64-macO.patch

@@ -243,13 +243,12 @@ index 8e03c77997..3b124c4b7b 100644
          include_dirs = []
          extra_compile_args = []
          extra_link_args = []
-@@ -2162,20 +2180,30 @@ class PyBuildExt(build_ext):
+@@ -2162,20 +2180,29 @@ class PyBuildExt(build_ext):
                       libraries=math_libs)
          self.extensions.extend([ext, ext_test])
  
 +        ffi_inc = sysconfig.get_config_var("LIBFFI_INCLUDEDIR")
 +        ffi_lib = None
-+        inc_dirs = self.compiler.include_dirs[:]
 +
          if host_platform == 'darwin':
 -            if '--with-system-ffi' not in sysconfig.get_config_var("CONFIG_ARGS"):
@@ -284,7 +283,7 @@ index 8e03c77997..3b124c4b7b 100644
              with open(ffi_h) as f:
                  for line in f:
                      line = line.strip()
-@@ -2186,15 +2214,22 @@ class PyBuildExt(build_ext):
+@@ -2186,15 +2213,22 @@ class PyBuildExt(build_ext):
                      ffi_inc = None
                      print('Header file {} does not define LIBFFI_H or '
                            'ffi_wrapper_h'.format(ffi_h))

plugins/python-build/share/python-build/patches/3.7.12/Python-3.7.12/0001-Port-ctypes-and-system-libffi-patches-for-arm64-macO.patch

@@ -243,15 +243,13 @@ index bf90600eaa..48ff120e9a 100644
          include_dirs = []
          extra_compile_args = []
          extra_link_args = []
-@@ -2018,30 +2036,49 @@ class PyBuildExt(build_ext):
+@@ -2018,30 +2036,47 @@ class PyBuildExt(build_ext):
                       libraries=['m'])
          self.extensions.extend([ext, ext_test])
  
 +        ffi_inc = sysconfig.get_config_var("LIBFFI_INCLUDEDIR")
 +        ffi_lib = None
 +
-+        #inc_dirs = self.inc_dirs.copy()
-+        inc_dirs = self.compiler.include_dirs[:]
          if host_platform == 'darwin':
 -            if '--with-system-ffi' not in sysconfig.get_config_var("CONFIG_ARGS"):
 +            if not self.use_system_libffi: