build/ruby: use notmuch configure script values for shared lib

This is supposed to help build on systems like MacOS with different
conventions for naming shared libraries. We have already computed the
relevant names, so doing it again in ruby seems like a bad idea.

bindings/Makefile.local

@@ -3,10 +3,11 @@
 dir := bindings
 
 # force the shared library to be built
-ruby-bindings: lib/libnotmuch.so
+ruby-bindings: lib/$(LINKER_NAME)
 ifeq ($(HAVE_RUBY_DEV),1)
 	cd $(dir)/ruby && \
 		EXTRA_LDFLAGS="$(NO_UNDEFINED_LDFLAGS)" \
+		LIBNOTMUCH="../../lib/$(LINKER_NAME)" \
 		ruby extconf.rb --vendor
 	$(MAKE) -C $(dir)/ruby
 else

bindings/ruby/extconf.rb

@@ -14,19 +14,12 @@ if ENV['EXTRA_LDFLAGS']
   $LDFLAGS += " " + ENV['EXTRA_LDFLAGS']
 end
 
-def have_local_library(lib, path, func, headers = nil)
-  checking_for checking_message(func, lib) do
-    lib = File.join(path, lib)
-    if try_func(func, lib, headers)
-      $LOCAL_LIBS += lib
-    end
-  end
-end
-
-if not have_local_library('libnotmuch.so', dir, 'notmuch_database_create', 'notmuch.h')
+if not ENV['LIBNOTMUCH']
   exit 1
 end
 
+$LOCAL_LIBS += ENV['LIBNOTMUCH']
+
 # Create Makefile
 dir_config('notmuch')
 create_makefile('notmuch')