Honor $(prefix) set in config.mak* when defining ETC_GIT* and sysconfdir

Definitions of ETC_GITCONFIG, ETC_GITATTRIBUTES and sysconfdir depend on
value of prefix. As prefix can be changed in config.mak.autogen, all if
blocks with conditions based on prefix should be placed after the file
is included in Makefile.

Signed-off-by: Kacper Kornet <draenog@pld-linux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -291,15 +291,8 @@ sharedir = $(prefix)/share
 gitwebdir = $(sharedir)/gitweb
 template_dir = share/git-core/templates
 htmldir = share/doc/git-doc
-ifeq ($(prefix),/usr)
-sysconfdir = /etc
-ETC_GITCONFIG = $(sysconfdir)/gitconfig
-ETC_GITATTRIBUTES = $(sysconfdir)/gitattributes
-else
-sysconfdir = $(prefix)/etc
-ETC_GITCONFIG = etc/gitconfig
-ETC_GITATTRIBUTES = etc/gitattributes
-endif
+ETC_GITCONFIG = $(git_etcdir)/gitconfig
+ETC_GITATTRIBUTES = $(git_etcdir)/gitattributes
 lib = lib
 # DESTDIR=
 pathsep = :
@@ -1196,6 +1189,14 @@ endif
 -include config.mak.autogen
 -include config.mak
 
+ifeq ($(prefix),/usr)
+sysconfdir = /etc
+git_etcdir = /etc
+else
+sysconfdir = $(prefix)/etc
+git_etcdir = etc
+endif
+
 ifdef CHECK_HEADER_DEPENDENCIES
 COMPUTE_HEADER_DEPENDENCIES =
 USE_COMPUTED_HEADER_DEPENDENCIES =