Makefile: move the last MinGW specific settings to the appropriate place

If we ever hope to use plain upstream git.git in msysGit, we need to
play nice with other platforms.  So move the MinGW specific settings
where they do not affect anybody else.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Makefile

@@ -191,7 +191,7 @@ STRIP ?= strip
 # runtime figures out where they are based on the path to the executable.
 # This can help installing the suite in a relocatable way.
 
-prefix =
+prefix = $(HOME)
 bindir_relative = bin
 bindir = $(prefix)/$(bindir_relative)
 mandir = share/man
@@ -235,7 +235,7 @@ AR = ar
 RM = rm -f
 TAR = tar
 FIND = find
-INSTALL = /bin/install
+INSTALL = install
 RPMBUILD = rpmbuild
 TCL_PATH = tclsh
 TCLTK_PATH = wish
@@ -592,7 +592,7 @@ BUILTIN_OBJS += builtin-verify-tag.o
 BUILTIN_OBJS += builtin-write-tree.o
 
 GITLIBS = $(LIB_FILE) $(XDIFF_LIB)
-EXTLIBS = /mingw/lib/libz.a
+EXTLIBS =
 
 #
 # Platform specific tweaks
@@ -779,6 +779,9 @@ ifneq (,$(findstring CYGWIN,$(uname_S)))
 	COMPAT_OBJS += compat/cygwin.o
 endif
 ifneq (,$(findstring MINGW,$(uname_S)))
+	prefix =
+	INSTALL = /bin/install
+	EXTLIBS += /mingw/lib/libz.a
 	NO_MMAP = YesPlease
 	NO_PREAD = YesPlease
 	NO_OPENSSL = YesPlease