better instructions for first-time users

Summary:
public
The instructions to install opam from the web on linux were not enough:
you need to install opam somewhere on the PATH. Moreover, the wget
command could erase "opam" if launched from the infer repo, which
already has an "opam" file.

./build-infer.sh's "opam init" command would fail if ocaml was not already
installed, instead of installing a default compiler.

Finally, check that ocamlfind is installed using the same logic as other
ocamlfind packages, to get an error message explaining how to get it
using opam if it's not installed, instead of just a failure.

Reviewed By: jeremydubreil

Differential Revision: D2760678

fb-gh-sync-id: 678178e

INSTALL.md

@@ -106,6 +106,8 @@ compile Infer on a few Linux distributions.
 sudo apt-get update
 sudo apt-get upgrade
 sudo apt-get install -y \
+  autoconf \
+  automake \
   build-essential \
   git \
   libgmp-dev \
@@ -125,6 +127,8 @@ sudo add-apt-repository ppa:ubuntu-toolchain-r/test
 sudo apt-get update
 sudo apt-get upgrade
 sudo apt-get install -y \
+  autoconf \
+  automake \
   build-essential \
   g++-4.8 \
   gcc-4.8 \
@@ -148,8 +152,10 @@ Unfortunately, the version of opam that ships with some Linux
 distributions is broken, so you'll have to get it from the web:
 
 ```sh
-wget -O opam https://github.com/ocaml/opam/releases/download/1.2.2/opam-1.2.2-x86_64-Linux
+wget https://github.com/ocaml/opam/releases/download/1.2.2/opam-1.2.2-x86_64-Linux
-chmod +x opam
+chmod +x opam-1.2.2-x86_64-Linux
+sudo cp opam-1.2.2-x86_64-Linux /usr/local/bin/opam
+opam init --comp=4.02.3
 ```
 
 Alternatively, follow the instructions [from the opam

build-infer.sh

@@ -96,15 +96,19 @@ check_installed () {
 echo "initializing opam... "
 check_installed opam
 
-# opam is noisy
+# if the first command doesn't succeed it means that ocaml is not even
-opam init --no-setup --yes > /dev/null
+# installed on the system, so we have to pick a compiler version ourselves
+#
+# opam is noisy, so we silence the first invocation which typically would be a
+# no-op
+opam init --no-setup --yes > /dev/null || \
+  opam init --no-setup --yes --comp=4.02.3
+
 eval $(opam config env)
 
 echo "preparing build... "
 if [ ! -f .release ]; then
   ./autogen.sh > /dev/null
-elif [ ! -d .git ] && [ ! -f .release ]; then
-  git submodule update --init --recursive
 fi
 
 TARGETS=""

configure.ac

@@ -149,7 +149,6 @@ AC_ASSERT_PROG([ocamlc], [$OCAMLC])
 AC_ASSERT_PROG([ocamlopt], [$OCAMLOPT])
 AC_ASSERT_PROG([ocamlbuild], [$OCAMLBUILD])
 AC_PROG_FINDLIB
-AC_ASSERT_PROG([ocamlfind], [$OCAMLFIND])
 AC_PROG_OCAMLLEX
 AC_ASSERT_PROG([ocamllex], [$OCAMLLEX])
 AC_PROG_OCAMLYACC

m4/ac_assert_ocaml_pkg.m4

@@ -9,16 +9,33 @@ dnl AC_ASSERT_OCAML_PKG([pkg_name], [pkg_version])
 dnl
 dnl fails if $pkg_name is not installed; also fails if it is not
 dnl installed at version $pkg_version if specified
+dnl
+dnl assumes that AC_PROG_FINDLIB has been called
 AC_DEFUN([AC_ASSERT_OCAML_PKG],
 [dnl
   AC_CHECK_OCAML_PKG_PATH([$1], [$2], [$3])
 
   unset has_pkg
   unset pkg
+  unset version
 
+  AS_IF([test "$OCAMLFIND" = "no"], [dnl
+    # trick to detect if ocamlfind is correctly installed and give the
+    # right opam instructions to install it if not since they're the
+    # same as when any other opam package is missing.
+    has_pkg=no
+    pkg=ocamlfind
+    version=
+  ], [dnl
     has_pkg=$AS_TR_SH[OCAML_PKG_$1]
+    pkg=$1
+    version=
+    AS_IF([test "x$3" = "x"], [dnl
+      version=" version $3"
+    ])
+  ])
   AS_IF([test "$has_pkg" = "no"], [dnl
-    AC_MSG_ERROR([missing dependency: $1$version.
+    AC_MSG_ERROR([missing dependency: $pkg$version.
 
 If you are using opam, please run