Issue #15819: Remove old unconditional -IInclude option

A newer instance of this option, enabled in the configure script when
building outside the source tree, made this redundant.

Makefile.pre.in

@@ -90,7 +90,7 @@ PY_CFLAGS_NODIST=$(CONFIGURE_CFLAGS_NODIST) $(CFLAGS_NODIST)
 # Both CPPFLAGS and LDFLAGS need to contain the shell's value for setup.py to
 # be able to build extension modules using the directories specified in the
 # environment variables
-PY_CPPFLAGS=	$(BASECPPFLAGS) -I. -IInclude -I$(srcdir)/Include $(CONFIGURE_CPPFLAGS) $(CPPFLAGS)
+PY_CPPFLAGS=	$(BASECPPFLAGS) -I. -I$(srcdir)/Include $(CONFIGURE_CPPFLAGS) $(CPPFLAGS)
 PY_LDFLAGS=	$(CONFIGURE_LDFLAGS) $(LDFLAGS)
 NO_AS_NEEDED=	@NO_AS_NEEDED@
 LDLAST=		@LDLAST@

Misc/NEWS

@@ -328,6 +328,9 @@ Build
 - Issue #28066: Fix the logic that searches build directories for generated
   include files when building outside the source tree.
 
+- Issue #15819: Remove redundant include search directory option for building
+  outside the source tree.
+
 - Issue #27566: Fix clean target in freeze makefile (patch by Lisa Roach)
 
 - Issue #27705: Update message in validate_ucrtbase.py