Trac #29617: check for ecl-config instead of ecl.

The plan for ecl's ./configure check is to build a test program using
a header that defines its version, and to check that version against
the required one. If those headers are installed in an uncommon
location, the ecl-config program exists to provide the appropriate
include path(s). Since we will need those include path(s) anyway, we
might as well check for ecl-config instead of ecl itself.

build/pkgs/ecl/spkg-configure.m4

@@ -1,8 +1,8 @@
 SAGE_SPKG_CONFIGURE([ecl], [
     SAGE_SPKG_DEPCHECK([gc gmp mpir], [
-	        AC_PATH_PROG([ECL], [ecl])
-	        AS_IF([test x$ECL = x], [
-	           AC_MSG_NOTICE([ecl not found. Installing ecl])
+	        AC_PATH_PROG([ECL_CONFIG], [ecl-config])
+	        AS_IF([test x$ECL_CONFIG = x], [
+	           AC_MSG_NOTICE([ecl-config not found. Installing ecl])
 	           sage_spkg_install_ecl=yes])
     ])
 ])