Package variables don't work in available field

Package variables like `conf-ios:arch` don't work in the `available` field. See the "Now let's get to the funny details..." section in ocaml/opam#2537 (comment). The limitation is also mentioned in the opam docs: https://opam.ocaml.org/doc/Manual.html#opamfield-available.

I'm not sure of the right fix but here's an idea. Change `available` field to rely on a global variable rather than a package variable. I defined a global variable `conf-ios-arch` at my command line like this:

```
opam config set conf-ios-arch "amd64"
```

README.md

@@ -31,23 +31,27 @@ Pin some prerequisite packages that don't yet have fixes merged upstream:
 
 Configure the compiler for 32-bit ARM:
 
-    ARCH=arm SUBARCH=armv7s PLATFORM=iPhoneOS SDK=9.3 VER=8.0 \
-      opam install conf-ios
+    (export ARCH=arm SUBARCH=armv7s PLATFORM=iPhoneOS SDK=11.2 VER=8.0 \
+      && opam config set conf-ios-arch $ARCH \
+      && opam install conf-ios)
 
 ... for 64-bit ARM
 
-    ARCH=arm64 SUBARCH=arm64 PLATFORM=iPhoneOS SDK=9.3 VER=8.0 \
-      opam install conf-ios
+    (export ARCH=arm64 SUBARCH=arm64 PLATFORM=iPhoneOS SDK=11.2 VER=8.0 \
+      && opam config set conf-ios-arch $ARCH \
+      && opam install conf-ios)
 
 ... for 32-bit x86:
 
-    ARCH=i386 SUBARCH=i386 PLATFORM=iPhoneSimulator SDK=9.3 VER=8.0 \
-      opam install conf-ios
+    (export ARCH=i386 SUBARCH=i386 PLATFORM=iPhoneSimulator SDK=11.2 VER=8.0 \
+      && opam config set conf-ios-arch $ARCH \
+      && opam install conf-ios)
 
 ... for 64-bit x86:
 
-    ARCH=amd64 SUBARCH=x86_64 PLATFORM=iPhoneSimulator SDK=9.3 VER=8.0 \
-      opam install conf-ios
+    (export ARCH=amd64 SUBARCH=x86_64 PLATFORM=iPhoneSimulator SDK=11.2 VER=8.0 \
+      && opam config set conf-ios-arch $ARCH \
+      && opam install conf-ios)
 
 Some options can be further tweaked:
 

packages/ocaml-ios32.4.04.0/opam

@@ -22,4 +22,4 @@ depopts: [
 conflicts: [
   "ocaml-ios64"
 ]
-available: [ ocaml-version = "4.04.0" & (conf-ios:arch = "arm" | conf-ios:arch = "i386") ]
+available: [ ocaml-version = "4.04.0" & (conf-ios-arch = "arm" | conf-ios-arch = "i386") ]

packages/ocaml-ios64.4.04.0/opam

@@ -22,4 +22,4 @@ depopts: [
 conflicts: [
   "ocaml-ios32"
 ]
-available: [ ocaml-version = "4.04.0" & (conf-ios:arch = "arm64" | conf-ios:arch = "amd64") ]
+available: [ ocaml-version = "4.04.0" & (conf-ios-arch = "arm64" | conf-ios-arch = "amd64") ]