tcl: disable threads.

* the exec implementation tries to use a combination of fork/exec with
a monitoring thread, which isn't conformant.

dev-lang/tcl/tcl-8.5.18.recipe

@@ -8,7 +8,7 @@ is truly cross platform, easily deployed and highly extensible."
 HOMEPAGE="http://www.tcl.tk"
 COPYRIGHT="Regents of the University of California, Sun Microsystems, Inc., Scriptics Corporation, ActiveState Corporation and other parties"
 LICENSE="BSD (2-clause)"
-REVISION="3"
+REVISION="4"
 SOURCE_URI="http://sourceforge.net/projects/tcl/files/Tcl/$portVersion/tcl$portVersion-src.tar.gz"
 CHECKSUM_SHA256="032be57a607bdf252135b52fac9e3a7016e526242374ac7637b083ecc4c5d3c9"
 SOURCE_DIR="tcl$portVersion"
@@ -52,7 +52,7 @@ BUILD()
 	cd unix
 	autoconf -f
 	runConfigure ./configure \
-		--enable-threads --enable-man-symlinks
+		--enable-man-symlinks
 	make
 }