configure: fix zlib and lz4 submodule checks

Conditionals to skip the submodule checks were using the wrong
variable names, so the checks were always skipped. The correct
behavior is to perform the check unless given `ZLIB=<zlib>` or
`LZ4=<lz4>`, as applicable.

configure

@@ -897,13 +897,13 @@ if [ ! -f "$srcdir"/nanopass/nanopass.ss ] ; then
     submod_instructions 'Source in "nanopass" is missing'
 fi
 
-if [ "${zlibDep}" != "" ] ; then
+if [ "${zlibLib}" = "" ] ; then
     if [ ! -f "$srcdir"/zlib/configure ] ; then
         submod_instructions 'Source in "zlib" is missing'
     fi
 fi
 
-if [ "${LZ4Dep}" != "" ] ; then
+if [ "${LZ4Lib}" = "" ] ; then
     if [ ! -f "$srcdir"/lz4/lib/Makefile ] ; then
         submod_instructions 'Source in "lz4" is missing'
     fi
@@ -1083,7 +1083,7 @@ cp "$srcdir"/makefiles/buildmain.zuo main.zuo
 # Some idea, but in the workarea, so it refers to "workarea.zuo" here:
 cp "$srcdir"/makefiles/workmain.zuo $w/main.zuo
 
-# The content of "$w/Makefile" records configuration decisions,
+# The content of "$w/Mf-config" records configuration decisions,
 # and the Zuo build script takes it from there
 cat > $w/Mf-config << END
 srcdir=$srcdir