Support overriding python3 detection

Also mention it in the README.

README.md

@@ -53,6 +53,16 @@ Requirements
 
 - Optional, required for Unicode support: ICU (Ubuntu: package `libicu-dev`).
 
+Python 3 Support
+----------------
+
+If your Vim is compiled against Python 3 instead (`+python3` flag) the install
+script should detect this in most cases and everything should just work. You may
+need different python headers installed (e.g. `python3-dev` on Ubuntu).
+
+If the detection does not work for any reason you can set `PY3=ON` or `PY3=OFF`
+as appropriate when running `./install.sh` to override it.
+
 Installation
 ------------
 

install.sh

@@ -4,10 +4,12 @@ set -o errexit
 set -o nounset
 set -o pipefail
 
-if vim --version | grep -q +python3; then
-    PY3="ON"
-else
-    PY3="OFF"
+if [ -z ${PY3+x} ]; then
+    if vim --version | grep -q +python3; then
+        PY3="ON"
+    else
+        PY3="OFF"
+    fi
 fi
 
 mkdir -p build