Issue #1584: Provide options to override default search paths for Tcl and Tk

when building _tkinter.  configure has two new options; if used, both must
be specified:

  ./configure \
      --with-tcltk-includes="-I/opt/local/include" \
      --with-tcltk-libs="-L/opt/local/lib -ltcl8.5 -ltk8.5"

In addition, the options can be overridden with make:

   make \
       TCLTK_INCLUDES="-I/opt/local/include" \
       TCLTK_LIBS="-L/opt/local/lib -ltcl8.6 -ltk8.6"

setup.py

@@ -1538,6 +1538,41 @@ class PyBuildExt(build_ext):
 
         return missing
 
+    def detect_tkinter_explicitly(self):
+        # Build _tkinter using explicit locations for Tcl/Tk.
+        #
+        # This is enabled when both arguments are given to ./configure:
+        #
+        #     --with-tcltk-includes="-I/path/to/tclincludes \
+        #                            -I/path/to/tkincludes"
+        #     --with-tcltk-libs="-L/path/to/tcllibs -ltclm.n \
+        #                        -L/path/to/tklibs -ltkm.n"
+        #
+        # These values can also be specified or overriden via make:
+        #    make TCLTK_INCLUDES="..." TCLTK_LIBS="..."
+        #
+        # This can be useful for building and testing tkinter with multiple
+        # versions of Tcl/Tk.  Note that a build of Tk depends on a particular
+        # build of Tcl so you need to specify both arguments and use care when
+        # overriding.
+
+        # The _TCLTK variables are created in the Makefile sharedmods target.
+        tcltk_includes = os.environ.get('_TCLTK_INCLUDES')
+        tcltk_libs = os.environ.get('_TCLTK_LIBS')
+        if not (tcltk_includes and tcltk_libs):
+            # Resume default configuration search.
+            return 0
+
+        extra_compile_args = tcltk_includes.split()
+        extra_link_args = tcltk_libs.split()
+        ext = Extension('_tkinter', ['_tkinter.c', 'tkappinit.c'],
+                        define_macros=[('WITH_APPINIT', 1)],
+                        extra_compile_args = extra_compile_args,
+                        extra_link_args = extra_link_args,
+                        )
+        self.extensions.append(ext)
+        return 1
+
     def detect_tkinter_darwin(self, inc_dirs, lib_dirs):
         # The _tkinter module, using frameworks. Since frameworks are quite
         # different the UNIX search logic is not sharable.
@@ -1627,10 +1662,16 @@ class PyBuildExt(build_ext):
         self.extensions.append(ext)
         return 1
 
-
     def detect_tkinter(self, inc_dirs, lib_dirs):
         # The _tkinter module.
 
+        # Check whether --with-tcltk-includes and --with-tcltk-libs were
+        # configured or passed into the make target.  If so, use these values
+        # to build tkinter and bypass the searches for Tcl and TK in standard
+        # locations.
+        if self.detect_tkinter_explicitly():
+            return
+
         # Rather than complicate the code below, detecting and building
         # AquaTk is a separate method. Only one Tkinter will be built on
         # Darwin - either AquaTk, if it is found, or X11 based Tk.