Add .build-var zle widget

Press Ctrl-B to fuzzy find all configuration CMake variables for the
current build directory. Once a variable is selected, rewrite the
command-line to set the selected CMake variable.

build/build.plugin.zsh

@@ -207,3 +207,23 @@ build-debug() {
   local target=$1; shift 1
   eval build $target && debug ~build/bin/$target "$@"
 }
+
+# Fuzzy find CMake variables, select one to set the variable via a command.
+.build-var() {
+  if [[ ! -n $build_dir ]]; then
+    build-dir
+  fi
+  local var=$(
+    cat $build_dir/CMakeCache.txt |
+    grep --color=never -Ex '^\w+:\w+=.*$' |
+    fzf --layout=reverse --info=hidden --border=rounded \
+        --cycle --height=50%
+  )
+  if [[ -n "$var" ]]; then
+    BUFFER="cmake -B\$build_dir -D$var"
+    zle end-of-line
+  fi
+  zle reset-prompt
+}
+zle -N .build-var
+bindkey '^B' .build-var