search for three variants
-lBLT (for debian and gentoo)
-lBLT24 (for redhat and centos)
-lBLT25 (might be usefull for someone else)
The advertised configure incantation is now:
configure --enable-shared=yes --with-tcl=/usr/lib/tcl8.6
In case you need a special "BLT" you can still use
LIBS=-lBLTfoobar configure --enable-shared=yes --with-tcl=/usr/lib/tcl8.6
to override the automatic search