Patch by Chris Waters sets paths which are normally point to a directory
with Tcl sources to a subdirectory of /usr/include/tcl8.6 (debian/rules
puts private Tcl headers there).

--- a/unix/tclConfig.sh.in
+++ b/unix/tclConfig.sh.in
@@ -100,7 +100,7 @@
 
 # String to pass to linker to pick up the Tcl library from its
 # build directory.
-TCL_BUILD_LIB_SPEC='@TCL_BUILD_LIB_SPEC@'
+TCL_BUILD_LIB_SPEC='@TCL_LIB_SPEC@'
 
 # String to pass to linker to pick up the Tcl library from its
 # installed directory.
@@ -135,7 +135,7 @@
 # different place than the directory containing the source files, this
 # points to the location of the sources, not the location where Tcl was
 # compiled.
-TCL_SRC_DIR='@TCL_SRC_DIR@'
+TCL_SRC_DIR='@includedir@/tcl-private'
 
 # List of standard directories in which to look for packages during
 # "package require" commands.  Contains the "prefix" directory plus also
@@ -153,14 +153,14 @@
 
 # String to pass to linker to pick up the Tcl stub library from its
 # build directory.
-TCL_BUILD_STUB_LIB_SPEC='@TCL_BUILD_STUB_LIB_SPEC@'
+TCL_BUILD_STUB_LIB_SPEC='@TCL_STUB_LIB_SPEC@'
 
 # String to pass to linker to pick up the Tcl stub library from its
 # installed directory.
 TCL_STUB_LIB_SPEC='@TCL_STUB_LIB_SPEC@'
 
 # Path to the Tcl stub library in the build directory.
-TCL_BUILD_STUB_LIB_PATH='@TCL_BUILD_STUB_LIB_PATH@'
+TCL_BUILD_STUB_LIB_PATH='@TCL_STUB_LIB_PATH@'
 
 # Path to the Tcl stub library in the install directory.
 TCL_STUB_LIB_PATH='@TCL_STUB_LIB_PATH@'