This resolves https://bugs.gentoo.org/731120
--- a/unix/configure.in
+++ b/unix/configure.in
@@ -895,7 +895,7 @@ TCL_BUILD_STUB_LIB_PATH="`pwd`/${TCL_STUB_LIB_FILE}"
 TCL_STUB_LIB_PATH="${TCL_STUB_LIB_DIR}/${TCL_STUB_LIB_FILE}"
 
 # Install time header dir can be set via --includedir
-eval "TCL_INCLUDE_SPEC=\"-I${includedir}\""
+eval "TCL_INCLUDE_SPEC=\"\""
 
 #------------------------------------------------------------------------
 # tclConfig.sh refers to this by a different name