configure fails to check for header supplied by libtool

[Mark Eackloff]

gnucash-1.8.11
configure runs with 0 return code.
Compile of src/gnc-module.c fails because it can't find ltdl.h.
configure.log says:
"configure:7031: checking if libtool supports shared libraries"
but libtool is not installed yet configure exits with 0 and subsequent
compile of gnc-module.c fails.