Don't skip the installer directory by default

this matches firefox behavior too.

mail/config/mozconfig

@@ -8,7 +8,6 @@ mk_add_options MOZ_THUNDERBIRD=1
 
 ac_add_options --enable-crypto
 ac_add_options --disable-mathml
-ac_add_options --disable-installer
 ac_add_options --disable-activex
 ac_add_options --disable-activex-scripting
 ac_add_options --disable-tests