Revert "Makefile.am: add -no-undefined for cygwin"

This reverts commit f49e0e5.

Turns out it wasn't harmless, and it breaks mac OS. Instead of adding
platform checks here, this flag is now included in the cygwin-specific
parts of the bitlbee pkg-config file, as of bitlbee commit b9c10a1a
(version string 3.5.1+20170529+develop+18-gb9c10a1a-git)

Author: dequis

steam/Makefile.am

@@ -30,4 +30,4 @@ steam_la_SOURCES = \
 	steam-util.h
 
 # Build the library as a module
-steam_la_LDFLAGS += -module -avoid-version -no-undefined
+steam_la_LDFLAGS += -module -avoid-version