configure: replace buggy AX_RUBY_EXT by a proper implementation

This actually allows to get the proper DLEXT expected by Ruby on
the target system.

Makefile.am

@@ -660,7 +660,7 @@ RDIR = bindings/ruby
 RDOC = $(RDIR)/doc.i
 RWRAP = $(RDIR)/classes_wrap.cpp
 ROBJ = $(RWRAP:.cpp=.o)
-REXT = $(RDIR)/sigrok.so
+REXT = $(RDIR)/sigrok.$(RUBY_DLEXT)
 
 $(RDOC): $(srcdir)/bindings/swig/doc.py $(CPPXMLDOC)
 	$(AM_V_GEN)$(PYTHON) $(srcdir)/bindings/swig/doc.py ruby $(CPPXMLDOC) > $@
@@ -683,8 +683,8 @@ $(REXT): $(ROBJ) @ORDER@ bindings/cxx/libsigrokcxx.la
 ruby-build: $(REXT)
 
 ruby-install: $(REXT)
-	$(INSTALL) -d $(prefix)/$(RBSIGROK_EXTDIR)
-	$(INSTALL) $< $(prefix)/$(RBSIGROK_EXTDIR)
+	$(INSTALL) -d $(DESTDIR)$(prefix)/$(RBSIGROK_EXTDIR)
+	$(INSTALL) $< $(DESTDIR)$(prefix)/$(RBSIGROK_EXTDIR)
 
 ruby-clean:
 	-$(AM_V_at)rm -fr $(RDIR)/doc

configure.ac

@@ -403,10 +403,16 @@ AS_IF([test "x$BINDINGS_CXX" = xyes],
 	[sr_ruby_missing=],
 	[sr_ruby_missing='C++ bindings'])
 
-AX_RUBY_EXT
+AC_PATH_PROGS(RUBY, ["${RUBY-ruby}"], [])
+AS_IF([test "x$RUBY" != x],
+	AC_MSG_CHECKING([for Ruby version])
+    [RUBY_VERSION=`$RUBY -e 'puts RUBY_VERSION'`]
+    AC_MSG_RESULT([$RUBY_VERSION])
+    [RUBY_DLEXT=`$RUBY -rrbconfig -e 'puts RbConfig::CONFIG[["DLEXT"]]'`]
+    AC_SUBST(RUBY_DLEXT))
 
 AS_IF([test "x$RUBY" = x],
-	[SR_APPEND([sr_ruby_missing], [', '], ['Ruby'])])
+	[SR_APPEND([sr_ruby_missing], [', '], [Ruby])])
 
 # Extract major and minor version number of the Ruby interpreter.
 sr_rbmajor=${RUBY_VERSION%%.*}
@@ -430,7 +436,6 @@ AS_IF([test "x$SWIG" = x],
 AS_IF([test -z "$sr_ruby_missing"],
 	[BINDINGS_RUBY=$enable_ruby], [BINDINGS_RUBY=no])
 AM_CONDITIONAL([BINDINGS_RUBY], [test "x$BINDINGS_RUBY" = xyes])
-# AC_SUBST(RUBY_EXT_DLEXT, $RUBY_EXT_DLEXT)
 
 ####################
 ##  Java bindings ##