Add DLL building example

PUBLISHED_FROM=20d24793d766ee289c6bde95bd5ad01202cd614b

examples/dll/Makefile

+# This Makefile builds Mongoose as a shared library on different architectures.
+# To pass a specific build options, use CFLAGS_EXTRA, for example:
+# make macos CFLAGS_EXTRA="-DMG_ENABLE_COAP"
+
+CFLAGS = -W -Wall -O2 $(CFLAGS_EXTRA)
+SRC = ../../mongoose.c
+
+all:
+	@echo "make <unix|windows> CFLAGS_EXTRA='-O2'"
+
+unix:
+	$(CC) $(SRC) -shared -o mongoose.so $(CFLAGS)
+
+windows:
+	cl $(SRC) /LD advapi32.lib $(CFLAGS_EXTRA)
+
+clean:
+	rm -rf *.so *.dll *.obj *.exe