Always apply -DHAVE_UNISTD_H

This commit also moves the CPPFLAGS config from Makefile, which is
autogenerated, to build_handwritten.yaml, which is literally
handwritten.

Makefile

@@ -2455,9 +2455,7 @@ PUBLIC_HEADERS_C += \
 LIBZ_OBJS = $(addprefix $(OBJDIR)/$(CONFIG)/, $(addsuffix .o, $(basename $(LIBZ_SRC))))
 
 $(LIBZ_OBJS): CFLAGS += -fvisibility=hidden
-ifeq ($(SYSTEM),Darwin)
 $(LIBZ_OBJS): CPPFLAGS += -DHAVE_UNISTD_H
-endif
 
 $(LIBDIR)/$(CONFIG)/libz.a:  $(LIBZ_OBJS) 
 	$(E) "[AR]      Creating $@"

build_handwritten.yaml

@@ -216,6 +216,7 @@ defaults:
     LDFLAGS: -g
   zlib:
     CFLAGS: -fvisibility=hidden
+    CPPFLAGS: -DHAVE_UNISTD_H
 php_config_m4:
   deps:
   - grpc