Improve git-prompt compilation error message

prompt_fresh_setup

@@ -122,9 +122,10 @@ visible_length() {
 compile_git_prompt() {
   # Compile a simple C executable which parses the output of `git status
   # --procelain` to greatly decrease the time taken to render the prompt
-  if [ ! -d ~/.cache/zsh ]; then mkdir -p ~/.cache/zsh; fi
-  if [ ! -f ~/.cache/zsh/git-prompt ]; then
-    cc -x c -std=gnu99 -O3 -DNDEBUG -o ~/.cache/zsh/git-prompt - << EOF
+  local cache=~/.cache/zsh
+  if [ ! -d $cache ]; then mkdir -p $cache; fi
+  if [ ! -f $cache/git-prompt ]; then
+    cc -x c -std=gnu99 -O3 -DNDEBUG -o $cache/git-prompt - 2> /dev/null << EOF
 #include <stdio.h>
 #include <stdlib.h>
 
@@ -176,6 +177,9 @@ int main() {
   return 0;
 }
 EOF
+    if [ $? -ne 0 ]; then
+      echo "git prompt disabled, are the system development headers installed?"
+    fi
   fi
 }