Fix build git installers in v2.53.0 rc1 (#852)

This includes the fixes I had to make to get
https://github.com/microsoft/git/releases/tag/v2.53.0-rc0.vfs.0.0 to
build, plus a fix for an overlooked stale `GIT-VERSION-GEN`,

Author: dscho

contrib/credential/osxkeychain/Makefile

@@ -41,8 +41,8 @@ prefix ?= /usr/local
 gitexecdir ?= $(prefix)/libexec/git-core
 
 CC ?= gcc
-CFLAGS ?= -g -O2 -Wall -I../../.. $(BASIC_CFLAGS)
-LDFLAGS ?= $(BASIC_LDFLAGS) $(EXTLIBS)
+override CFLAGS += -g -O2 -Wall -I../../.. $(BASIC_CFLAGS)
+override LDFLAGS += $(BASIC_LDFLAGS) $(EXTLIBS)
 INSTALL ?= install
 RM ?= rm -f