[BUG] 'make install' may overwrite existing directories

Hi,

The project Makefile defines the install target as:

$ sed -n '3236,3245p' Makefile

install: all
	$(INSTALL) -d -m 755 '$(DESTDIR_SQ)$(bindir_SQ)'
	$(INSTALL) -d -m 755 '$(DESTDIR_SQ)$(gitexec_instdir_SQ)'
	$(INSTALL) $(INSTALL_STRIP) $(PROGRAMS) 
'$(DESTDIR_SQ)$(gitexec_instdir_SQ)'
	$(INSTALL) $(SCRIPTS) '$(DESTDIR_SQ)$(gitexec_instdir_SQ)'
	$(INSTALL) -m 644 $(SCRIPT_LIB) '$(DESTDIR_SQ)$(gitexec_instdir_SQ)'
	$(INSTALL) $(INSTALL_STRIP) $(install_bindir_xprograms) 
'$(DESTDIR_SQ)$(bindir_SQ)'
	$(INSTALL) $(BINDIR_PROGRAMS_NO_X) '$(DESTDIR_SQ)$(bindir_SQ)'

Notice that the directories are being installed even if they already exist. This may be problematic in case the existing directories have a different mode, or have SELinux security contexts, or any other characteristic that install(1) would overwrite.

A similar bug report in an NGINX project can be found here:

<https://github.com/nginx/unit/issues/769>

The two solutions available are:

- Make the install target be a dummy target that depends on the files 
being installed, and write proper make(1) rules for those targets.  This 
would be preferred.  You can check the Linux man-pages Makefile for an 
implementation of the 'install' target that does this.

- test(1) if the directory exists before actually creating it.  This is 
a simple workaround.

Cheers,

Alex