cygwin: Link many libraries with -no-undefined to create a dll

Only the Gb library relies on having undefined references to a
symbol that needs to be provided by the host application. For
all other libraries we can link with -no-undefined.
diff --git a/src/Makefile.am b/src/Makefile.am
index 081be96..e85f3e3 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -18,9 +18,9 @@
 
 if ENABLE_PLUGIN
 libosmocore_la_SOURCES += plugin.c
-libosmocore_la_LDFLAGS = -version-info $(LIBVERSION) $(LIBRARY_DL)
+libosmocore_la_LDFLAGS = -version-info $(LIBVERSION) $(LIBRARY_DL) -no-undefined
 else
-libosmocore_la_LDFLAGS = -version-info $(LIBVERSION)
+libosmocore_la_LDFLAGS = -version-info $(LIBVERSION) -no-undefined
 endif
 
 if ENABLE_TALLOC