When ‘--help’ is given, just run ‘man’ to show the manual page

I.e. do what git does.  I'm too lazy to keep the builtin help text up
to date :-)

Also add ‘--help’ to various commands that lacked it
(e.g. nix-collect-garbage).

src/nix-instantiate/Makefile.am

@@ -1,15 +1,10 @@
 bin_PROGRAMS = nix-instantiate
 
-nix_instantiate_SOURCES = nix-instantiate.cc help.txt
+nix_instantiate_SOURCES = nix-instantiate.cc
 nix_instantiate_LDADD = ../libmain/libmain.la ../libexpr/libexpr.la \
  ../libstore/libstore.la ../libutil/libutil.la \
  ../boost/format/libformat.la
 
-nix-instantiate.o: help.txt.hh
-
-%.txt.hh: %.txt
-	../bin2c/bin2c helpText < $< > $@ || (rm $@ && exit 1)
-
 AM_CXXFLAGS = \
  -I$(srcdir)/.. -I$(srcdir)/../libutil -I$(srcdir)/../libstore \
  -I$(srcdir)/../libexpr -I$(srcdir)/../libmain -I../libexpr

src/nix-instantiate/help.txt

@@ -1,31 +0,0 @@
-Usage: nix-instantiate [OPTIONS...] [FILES...]
-
-`nix-instantiate' turns Nix expressions into store derivations. 
-
-The argument `-' may be specified to read a Nix expression from
-standard input.
-
-Options:
-
-  --version: output version information
-  --help: display help
-
-  --verbose / -v: verbose operation (may be repeated)
-
-  --eval-only: evaluate and print resulting term; do not instantiate
-  --parse-only: parse and print abstract syntax tree
-
-  --attr / -A PATH: select an attribute from the top-level expression
-
-  --add-root: add garbage collector roots for the result
-
-For --eval-only / --parse-only:
-
-  --xml: print an XML representation of the abstract syntax tree
-  --no-location: don't provide source location information in the
-    output XML tree
-
-For --eval-only:
-
-  --strict: compute attributes and list elements, rather than being
-    lazy

src/nix-instantiate/nix-instantiate.cc

@@ -8,7 +8,6 @@
 #include "util.hh"
 #include "store-api.hh"
 #include "common-opts.hh"
-#include "help.txt.hh"
 
 #include <map>
 #include <iostream>
@@ -19,7 +18,7 @@ using namespace nix;
 
 void printHelp()
 {
-    std::cout << string((char *) helpText);
+    showManPage("nix-instantiate");
 }