Postgres is ignoring TCL_INCLUDE_SPEC

I just found that Postgres fails to build against Tcl/Tk installed in a
nonstandard place (specifically, I'd put tcl 8.4.1 into /opt/tcl8.4).
The problem appears to be that the Makefiles ignore TCL_INCLUDE_SPEC,
which tclConfig.sh correctly sets up:

# String to pass to the compiler so that an extension can
# find installed Tcl headers.
TCL_INCLUDE_SPEC='-I/opt/tcl8.4/include'

Is there a reason for ignoring this, or is it just an oversight?
        regards, tom lane