Only add aliases when the target command exists

Previously aliases for commands were added without checking if the
command being aliased existed first, this is now not the case.

zshrc

@@ -130,20 +130,27 @@ fi
 
 # Aliases
 alias grep='grep --color=always'
-alias cninja='cmake -GNinja -DCMAKE_EXPORT_COMPILE_COMMNADS=ON'
-alias ssh='TERM=xterm-256color ssh'
+which cmake &> /dev/null && \
+  alias cninja='cmake -GNinja -DCMAKE_EXPORT_COMPILE_COMMNADS=ON'
+which ssh &> /dev/null && \
+  alias ssh='TERM=xterm-256color ssh'
 
 case `uname` in
   Linux)
     [ "$TMUX" = "" ] && \
-      alias cls="printf '\ec'" || alias cls="clear && printf '\e[3J'"
+      alias cls="printf '\ec'" || \
+      alias cls="clear && printf '\e[3J'"
     alias ls='ls -F --color=auto'
-    alias debug='cgdb --args'
+    which cgdb &> /dev/null && \
+      alias debug='cgdb --args' || \
+      which gdb &> /dev/null && \
+        alias debug='gdb --args'
     ;;
   Darwin)
     alias cls="clear && printf '\e[3J'"
     alias ls='ls -GFh'
-    alias debug='lldb --'
+    which lldb &> /dev/null && \
+      alias debug='lldb --'
     ;;
 esac