[email protected]: 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).

Author: Mitchell Hentges

Formula/[email protected]

@@ -4,6 +4,7 @@ class PythonAT38 < Formula
   url "https://www.python.org/ftp/python/3.8.7/Python-3.8.7.tar.xz"
   sha256 "ddcc1df16bb5b87aa42ec5d20a5b902f2d088caa269b28e01590f97a798ec50a"
   license "Python-2.0"
+  revision 1
 
   livecheck do
     url "https://www.python.org/ftp/python/"
@@ -100,19 +101,26 @@ def install
       --with-openssl=#{Formula["[email protected]"].opt_prefix}
     ]
 
-    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}"
       # For the Xlib.h, Python needs this header dir with the system Tk
       # Yep, this needs the absolute path where zlib needed a path relative
       # to the SDK.
-      cflags << "-I#{MacOS.sdk_path}/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers"
+      cflags << "-isystem #{MacOS.sdk_path}/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers"
     end
     # Avoid linking to libgcc https://mail.python.org/pipermail/python-dev/2012-February/116205.html
     args << "MACOSX_DEPLOYMENT_TARGET=#{MacOS.version}"
@@ -139,7 +147,9 @@ 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