build-dir now exports build_dir envvar

In addition to setting the `~build` hashed directory, also export the
`build_dir` environment variable to enable commands like this:

```
cmake . -B$build_dir
```

build/build.plugin.zsh

@@ -7,7 +7,10 @@ alias build="build-dir --build"
 # Detect installed debugger and set the `debug` alias to debug a program with
 # command line arguments.
 if [ `uname` = Linux ]; then
-  if which cgdb &> /dev/null; then
+  if [[ "`vim --version`" =~ "^VIM - Vi IMproved 8\.1.*$" ]]; then
+    function vimdebug() { vim "+packadd termdebug" "+TermdebugCommand $*" }
+    alias debug='vimdebug'
+  elif which cgdb &> /dev/null; then
     alias debug='cgdb --args'
   elif which gdb &> /dev/null; then
     alias debug='gdb --args'
@@ -39,7 +42,7 @@ EOF
   fi
   error() { echo "\e[31merror:\e[0m $1" }
   warning() { echo "\e[33mwarning:\e[0m $1" }
-  local build_dir
+  local local_build_dir
   if [[ ${#*} -gt 1 ]]; then
     echo $usage
     error "unexpected position arguments: ${*[2,${#*}]}"; return 1
@@ -47,24 +50,24 @@ EOF
     if [[ ! -d ${*[1]} ]]; then
       warning "directory not found: ${*[1]}"
     else
-      build_dir=${*[1]}
+      local_build_dir=${*[1]}
     fi
   fi
 
   # If <directory> was not set begin selection
-  if [[ -z $build_dir ]]; then
+  if [[ -z $local_build_dir ]]; then
     # Find build directories
-    local -a build_dirs
+    local -a local_build_dirs
     for entry in `ls -A`; do
       [ -d $entry ] && [[ $entry =~ build* ]] && \
-        build_dirs+=${entry/\//}
+        local_build_dirs+=${entry/\//}
     done
 
     # Interactively select a build directory if more than 1 found
     integer index=0
-    if [[ ${#build_dirs} -eq 0 ]]; then
+    if [[ ${#local_build_dirs} -eq 0 ]]; then
       error "no build directories found"; return 1
-    elif [[ ${#build_dirs} -gt 1 ]]; then
+    elif [[ ${#local_build_dirs} -gt 1 ]]; then
       zmodload zsh/curses && {
         # Get the size of the terminal
         local size=`stty size`
@@ -89,13 +92,13 @@ EOF
           zcurses string build-dir 'Select a build directory:'
 
           # Add the selections text
-          for (( i = 0; i < ${#build_dirs}; i++ )); do
+          for (( i = 0; i < ${#local_build_dirs}; i++ )); do
             integer line=$i+3
             zcurses move build-dir $line 1
             [[ $index -eq $i ]] &&
               zcurses string build-dir "* " ||
               zcurses string build-dir "  "
-            zcurses string build-dir ${build_dirs[$i+1]}
+            zcurses string build-dir ${local_build_dirs[$i+1]}
           done
 
           # Display the text the and wait for input
@@ -107,7 +110,7 @@ EOF
             (UP|k|$'\C-P')
               [[ $index -gt 0 ]] && index=$index-1 ;;
             (DOWN|j|$'\C-N')
-              [[ $index -lt ${#build_dirs}-1 ]] && index=$index+1 ;;
+              [[ $index -lt ${#local_build_dirs}-1 ]] && index=$index+1 ;;
             (ENTER|$'\n')
               break ;;
           esac
@@ -124,32 +127,33 @@ EOF
   # On success setup the build directory for use
   if [[ $? -eq 0 ]]; then
     # Set the build directory from selection if empty
-    [[ -z $build_dir ]] && \
-      build_dir=${build_dirs[$index+1]}
+    [[ -z $local_build_dir ]] && \
+      local_build_dir=${local_build_dirs[$index+1]}
 
     # If `build.ninja` exists in alias `ninja`, return.
     local build
-    [ -f $build_dir/build.ninja ] && \
-      build="ninja -C $build_dir"
+    [ -f $local_build_dir/build.ninja ] && \
+      build="ninja -C $local_build_dir"
 
     # If `Makefile` exists in alias `make`, return.
-    if [ -f $build_dir/Makefile ]; then
+    if [ -f $local_build_dir/Makefile ]; then
       [ `uname` = Darwin ] && \
         local cpu_count=`sysctl -n hw.ncpu` ||
         local cpu_count=`grep -c '^processor' /proc/cpuinfo`
-      build="make -j $cpu_count -C $build_dir"
+      build="make -j $cpu_count -C $local_build_dir"
     fi
 
     # If the build variable is not defined the command could not be determined
     if [ -z $build ]; then
-      warning "build command detection failed: $build_dir"
+      warning "build command detection failed: $local_build_dir"
       # Prompt user to enter a build command
       vared -p 'enter comand: ' build
     fi
 
     # Redefine the `build` alias and update the `~build` hash directory
     alias build="$build"
-    hash -d build=$build_dir
+    hash -d build=$local_build_dir
+    export build_dir=$local_build_dir
 
     # If `--build` is specified then evaluate the command.
     if [[ -n $do_build ]]; then