INSTALL: Describe dependency knobs from Makefile

We said that some of our dependencies were optional, but didn't say
how to turn them off.  Add information for that and mention where to
save the options close to the top of the file.

Also, standardize on both using quotes for the names of the dependencies
and tabs for indentation of the list.

Signed-off-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

INSTALL

@@ -13,6 +13,10 @@ that uses $prefix, the built results have some paths encoded,
 which are derived from $prefix, so "make all; make prefix=/usr
 install" would not work.
 
+The beginning of the Makefile documents many variables that affect the way
+git is built.  You can override them either from the command line, or in a
+config.mak file.
+
 Alternatively you can use autoconf generated ./configure script to
 set up install paths (via config.mak.autogen), so you can write instead
 
@@ -48,7 +52,9 @@ Issues of note:
 	export GIT_EXEC_PATH PATH GITPERLLIB
 
  - Git is reasonably self-sufficient, but does depend on a few external
-   programs and libraries:
+   programs and libraries.  Git can be used without most of them by adding
+   the approriate "NO_<LIBRARY>=YesPlease" to the make command line or
+   config.mak file.
 
 	- "zlib", the compression library. Git won't build without it.
 
@@ -59,25 +65,29 @@ Issues of note:
 
 	- "Perl" is needed to use some of the features (e.g. preparing a
 	  partial commit using "git add -i/-p", interacting with svn
-	  repositories with "git svn").
+	  repositories with "git svn").  If you can live without these, use
+	  NO_PERL.
 
-	- "openssl".  Unless you specify otherwise, you'll get the SHA1
-	  library from here.
+	- "openssl" library is used by git-imap-send to use IMAP over SSL.
+	  If you don't need it, use NO_OPENSSL.
 
-	  If you don't have openssl, you can use one of the SHA1 libraries
-	  that come with git (git includes one inspired by Mozilla's and a
-	  PowerPC optimized one too - see the Makefile).
+	  By default, git uses OpenSSL for SHA1 but it will use it's own
+	  library (inspired by Mozilla's) with either NO_OPENSSL or
+	  BLK_SHA1.  Also included is a version optimized for PowerPC
+	  (PPC_SHA1).
 
-	- libcurl library; git-http-fetch and git-fetch use them.  You
+	- "libcurl" library is used by git-http-fetch and git-fetch.  You
 	  might also want the "curl" executable for debugging purposes.
-	  If you do not use http transfer, you are probably OK if you
-	  do not have them.
+	  If you do not use http:// or https:// repositories, you do not
+	  have to have them (use NO_CURL).
 
-	- expat library; git-http-push uses it for remote lock
-	  management over DAV.  Similar to "curl" above, this is optional.
+	- "expat" library; git-http-push uses it for remote lock
+	  management over DAV.  Similar to "curl" above, this is optional
+	  (with NO_EXPAT).
 
-        - "wish", the Tcl/Tk windowing shell is used in gitk to show the
-          history graphically, and in git-gui.
+	- "wish", the Tcl/Tk windowing shell is used in gitk to show the
+	  history graphically, and in git-gui.  If you don't want gitk or
+	  git-gui, you can use NO_TCLTK.
 
  - Some platform specific issues are dealt with Makefile rules,
    but depending on your specific installation, you may not