I think it's because passenger has been "compiled" against the osx built-in apache headers. If you're using MAMP, you should compile it against MAMP's built-in apache headers.
I don't know how to specify which apache I'm using when I install it:
$sudo passenger-install-apache2-module
Welcome to the Phusion Passenger
Apache 2 module installer, v2.2.11.
This installer will guide you through
the entire installation process. It
shouldn't take more than 3 minutes in
total.
Here's what you can expect from the
installation process:
- The Apache 2 module will be installed for you.
- You'll learn how to configure Apache.
- You'll learn how to deploy a Ruby on Rails application.
Don't worry if anything goes wrong.
This installer will advise you on how
to solve any problems.
Press Enter to continue, or Ctrl-C to
abort.
Checking for required software...
- GNU C++ compiler... found at
/usr/bin/g++ * Ruby development
headers... found * OpenSSL support
for Ruby... found * RubyGems... found
- Rake... found at /usr/bin/rake * rack... found * Apache 2... found at
/usr/sbin/httpd * Apache 2
development headers... found at
/usr/sbin/apxs