Revert "Update support for Cygwin 2.10.0 - based on MSYS toolchain config."

This reverts commit 2b3a490f7cd45afc294aedad55196b7a5a89813f.

config/Makefile.cygwin

@@ -4,10 +4,9 @@ GLEW_DEST ?= /usr
 CC := cc
 LD := cc
 LN :=
-CFLAGS.EXTRA += -D_WIN32
 LDFLAGS.EXTRA =
 LIBDIR = $(GLEW_DEST)/lib
-LDFLAGS.GL = -lOpenGL32 -mwindows
+LDFLAGS.GL = -lGL -lX11
 LDFLAGS.STATIC = -Wl,-Bstatic
 LDFLAGS.DYNAMIC = -Wl,-Bdynamic
 WARN = -Wall -W