python@3.9: adjust flags to avoid package installation breakage

* Mac SDK include paths should be system include path.
* The brew include path should only be added for building Python,
  and shouldn't be re-used after (use FLAG_NODIST).

Closes #68528.

Signed-off-by: Carlo Cabrera <30379873+carlocab@users.noreply.github.com>

Formula/python@3.9.rb

@@ -124,14 +124,21 @@ def install
       args << "--with-system-ffi"
     end
 
-    cflags   = ["-I#{HOMEBREW_PREFIX}/include"]
-    ldflags  = ["-L#{HOMEBREW_PREFIX}/lib"]
-    cppflags = ["-I#{HOMEBREW_PREFIX}/include"]
+    # Python re-uses flags when building native modules.
+    # Since we don't want native modules prioritizing the brew
+    # include path, we move them to [C|LD]FLAGS_NODIST.
+    # Note: Changing CPPFLAGS causes issues with dbm, so we
+    # leave it as-is.
+    cflags         = []
+    cflags_nodist  = ["-I#{HOMEBREW_PREFIX}/include"]
+    ldflags        = []
+    ldflags_nodist = ["-L#{HOMEBREW_PREFIX}/lib"]
+    cppflags       = ["-I#{HOMEBREW_PREFIX}/include"]
 
     if MacOS.sdk_path_if_needed
       # Help Python's build system (setuptools/pip) to build things on SDK-based systems
       # The setup.py looks at "-isysroot" to get the sysroot (and not at --sysroot)
-      cflags  << "-isysroot #{MacOS.sdk_path}" << "-I#{MacOS.sdk_path}/usr/include"
+      cflags  << "-isysroot #{MacOS.sdk_path}"
       ldflags << "-isysroot #{MacOS.sdk_path}"
     end
     # Avoid linking to libgcc https://mail.python.org/pipermail/python-dev/2012-February/116205.html
@@ -162,7 +169,9 @@ def install
     end
 
     args << "CFLAGS=#{cflags.join(" ")}" unless cflags.empty?
+    args << "CFLAGS_NODIST=#{cflags_nodist.join(" ")}" unless cflags_nodist.empty?
     args << "LDFLAGS=#{ldflags.join(" ")}" unless ldflags.empty?
+    args << "LDFLAGS_NODIST=#{ldflags_nodist.join(" ")}" unless ldflags_nodist.empty?
     args << "CPPFLAGS=#{cppflags.join(" ")}" unless cppflags.empty?
 
     system "./configure", *args