Document MAMP problems.

doc/Users guide.txt

@@ -365,6 +365,27 @@ The default value is 'nobody'.
 
 == Troubleshooting ==
 
+=== Operating system-specific problems ===
+
+==== MacOS X: The installer cannot locate MAMP's Apache ====
+
+.Symptoms
+*******************************************************************************
+The installer finds Apache 2 development headers at `/Applications/MAMP/Library/bin/apxs`.
+However, Apache cannot be found. The installer also outputs the following error:
+------------------------------------
+cannot open /Applications/MAMP/Library/build/config_vars.mk:
+No such file or directory at /Applications/MAMP/Library/bin/apxs line 218.
+------------------------------------
+*******************************************************************************
+
+Your MAMP installation seems to be broken. In particular, 'config_vars.mk' is missing.
+Please read link:http://forum.mamp.info/viewtopic.php?t=1866[this forum topic] to learn how
+to fix this problem.
+
+See also link:http://code.google.com/p/phusion-passenger/issues/detail?id=12[this bug report].
+
+
 === Problems during installation ===
 
 [[installing_ruby_dev]]