installable: Only panic without sphinx if FISH_BUILD_DOCS=1

It's pretty annoying that this panics without sphinx, because the
install itself would be *working*.

So instead we tell the user that they need to clean or set
$FISH_BUILD_DOCS if they want to try again.

README.rst

@@ -185,11 +185,13 @@ This will place the binaries in ``~/.cargo/bin/``, but you can place them wherev
 
 This build won't have the HTML docs (``help`` will open the online version) or translations.
 
-It requires sphinx by default, if sphinx-build is not available you can build without man pages by running it with $FISH_BUILD_DOCS set to 0::
+It will try to build the man pages with sphinx-build. If that is not available and you would like to include man pages, you need to install it and retrigger the build script, e.g. by setting FISH_BUILD_DOCS=1::
 
-  FISH_BUILD_DOCS=0 cargo install --path .
+  FISH_BUILD_DOCS=1 cargo install --path .
 
-You can also link it statically (but not against glibc) and move it to other computers.
+Setting it to "0" disables the inclusion of man pages.
+
+You can also link this build statically (but not against glibc) and move it to other computers.
 
 Contributing Changes to the Code
 --------------------------------

build.rs

@@ -378,7 +378,11 @@ fn build_man(build_dir: &Path) {
     // which is unfortunate - but the docs are pretty important because they're also used for --help.
     match Command::new("sphinx-build").args(args).spawn() {
         Err(x) if x.kind() == std::io::ErrorKind::NotFound => {
-            panic!("Could not find sphinx-build to build man pages.\nInstall sphinx or disable building the docs by setting $FISH_BUILD_DOCS=0.");
+            if env::var("FISH_BUILD_DOCS") == Ok("1".to_string()) {
+                panic!("Could not find sphinx-build to build man pages.\nInstall sphinx or disable building the docs by setting $FISH_BUILD_DOCS=0.");
+            }
+            println!("cargo:warning=Cannot find sphinx-build to build man pages.");
+            println!("cargo:warning=If you install it now you need to run `cargo clean` and rebuild, or set $FISH_BUILD_DOCS=1 explicitly.");
         }
         Err(x) => {
             // Another error - permissions wrong etc