separate build targets for man and html documentation

This introduces new build targets "man" and "html" which allow building
the documentation in the respective formats separately. This helps
people with a partial documentation build chain: html pages can be built
without xmlto.

This is documented in INSTALL now, together with corrections: Before,
instructions in INSTALL would build man+html but install man only. Now
the instructions build and install both, and new and pre-existing
targets are explained.

Note that build targets "doc" and "man" correspond to install targets
"install-doc install-html" and "install-doc" respectively. This
inconsistency is not changed, in order to keep everyone's build scripts
from breaking.
Signed-off-by: NMichael J Gruber <michaeljgruber+gmane@fastmail.fm>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

INSTALL

@@ -6,7 +6,7 @@ will install the git programs in your own ~/bin/ directory.  If you want
 to do a global install, you can do
 
 	$ make prefix=/usr all doc info ;# as yourself
-	# make prefix=/usr install install-doc install-info ;# as root
+	# make prefix=/usr install install-doc install-html install-info ;# as root
 
 (or prefix=/usr/local, of course).  Just like any program suite
 that uses $prefix, the built results have some paths encoded,
@@ -19,7 +19,7 @@ set up install paths (via config.mak.autogen), so you can write instead
 	$ make configure ;# as yourself
 	$ ./configure --prefix=/usr ;# as yourself
 	$ make all doc ;# as yourself
-	# make install install-doc ;# as root
+	# make install install-doc install-html;# as root
 
 
 Issues of note:
@@ -89,13 +89,22 @@ Issues of note:
    inclined to install the tools, the default build target
    ("make all") does _not_ build them.
 
+   "make doc" builds documentation in man and html formats; there are
+   also "make man", "make html" and "make info". Note that "make html"
+   requires asciidoc, but not xmlto. "make man" (and thus make doc)
+   requires both.
+
+   "make install-doc" installs documentation in man format only; there
+   are also "make install-man", "make install-html" and "make
+   install-info".
+
    Building and installing the info file additionally requires
    makeinfo and docbook2X.  Version 0.8.3 is known to work.
 
    The documentation is written for AsciiDoc 7, but "make
    ASCIIDOC8=YesPlease doc" will let you format with AsciiDoc 8.
 
-   Alternatively, pre-formatted documentation are available in
+   Alternatively, pre-formatted documentation is available in
    "html" and "man" branches of the git repository itself.  For
    example, you could:
 

Makefile

@@ -1267,6 +1267,12 @@ $(XDIFF_LIB): $(XDIFF_OBJS)
 doc:
 	$(MAKE) -C Documentation all
 
+man:
+	$(MAKE) -C Documentation man
+
+html:
+	$(MAKE) -C Documentation html
+
 info:
 	$(MAKE) -C Documentation info