adms, polish autogen.sh and configure.ac

Allow to override the admsXml executable when invoking
  autogen.sh and configure.ac

ADMSXML=/foo/bar/baz/admsXml ./autogen.sh --adms
ADMSXML=/foo/bar/baz/admsXml ./configure --enable-adms
alternatively (and more advertised)
./configure ADMSXML=/foo/bar/baz/admsXml --enable-adms

The generated Makefiles remember the given path.
A relative path doesn't make sense of course.

autogen.sh

@@ -15,7 +15,7 @@ PROJECT=ngspice
 
 ADMSDIR=src/spicelib/devices/adms
 XMLPATH=src/spicelib/devices/adms/admst
-ADMSXML=admsXml
+ADMSXML=${ADMSXML:-admsXml}
 ADMS=0
 
 # Exit variable
@@ -96,7 +96,7 @@ check_autoconf()
 
 check_adms()
 {
-    (admsXml --version)   < /dev/null > /dev/null 2>&1 || {
+    ($ADMSXML --version) < /dev/null > /dev/null 2>&1 || {
         echo
 	echo "You must have admsXml installed to compile adms models."
 	echo "See http://mot-adms.sourceforge.net"

configure.ac

@@ -934,9 +934,9 @@ if test "x$enable_adms" = xyes ; then
     AC_MSG_RESULT([**********************************
 * ADMS  support is  experimental *
 **********************************])
-    AC_CHECK_PROGS([ADMSXML], [admsXml admsXml.exe], [no])
+    AC_CHECK_PROGS([ADMSXML], [admsXml admsXml.exe])
 
-    if test "x$ADMSXML" = xno; then
+    if test "x$ADMSXML" = x; then
         AC_MSG_ERROR([If you want Verilog-A models you should install admsXml])
     fi
     AC_DEFINE([ADMS], [1], [Support for Verilog-A models])
@@ -969,7 +969,6 @@ else
 
 fi
 
-AC_SUBST([ADMSXML])
 AC_SUBST([VLADEVDIR])
 AC_SUBST([VLADEV])
 AC_SUBST([NOTVLADEVDIR])

src/spicelib/devices/adms/admst/ngspiceMakefile.am.xml

@@ -109,7 +109,7 @@ DISTCLEANFILES = Makefile.am \\
 	\$(ADMSXMLINTERFACE)/ngspiceMODULEguesstopology.c.xml \\
 	\$(ADMSXMLINTERFACE)/ngspiceMODULE.hxx.xml \\
 	\$(ADMSXMLINTERFACE)/ngspiceMODULE.c.xml
-	admsXml -I\$(srcdir) -I\$(srcdir)/admsva \$< \\
+	\$(ADMSXML) -I\$(srcdir) -I\$(srcdir)/admsva \$< \\
 	-e \$(ADMSXMLINTERFACE)/ngspiceVersion.xml \\
 	-e \$(ADMSXMLINTERFACE)/ngspiceMODULEitf.h.xml \\
 	-e \$(ADMSXMLINTERFACE)/ngspiceMODULEinit.c.xml \\