The assets:precompile failure was my own from a misconfiguration related to the mongo driver in conf/initializers. Solved that. Still can't get Passenger compiled/installed:
** [out :: ip-address] Notice: /Exec[build_passenger]/returns: Command failed with status (1): [c++ -o buildout/agents/PassengerHelperAgent.o -Iext -Iext/common -Iext/libev -Iext/libeio -D_REENTRANT -I/usr/local/include -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wpointer-arith -Wwrite-strings -Wno-long-long -Wno-missing-field-initializers -fcommon -feliminate-unused-debug-symbols -feliminate-unused-debug-types -fvisibility=hidden -DVISIBILITY_ATTRIBUTE_SUPPORTED -Wno-attributes -ggdb -DHAS_ALLOCA_H -DHAVE_ACCEPT4 -DHAS_SFENCE -DHAS_LFENCE -DPASSENGER_DEBUG -DBOOST_DISABLE_ASSERTS -std=gnu++11 -Wno-unused-local-typedefs -DHASH_NAMESPACE="__gnu_cxx" -DHASH_MAP_HEADER="<hash_map>" -DHASH_MAP_CLASS="hash_map" -DHASH_FUN_H="<hash_fun.h>" -c ext/common/agents/HelperAgent/Main.cpp]
** [out :: ip-address] Notice: /Exec[build_passenger]/returns: /var/lib/gems/2.1.0/gems/passenger-4.0.58/build/cplusplus_support.rb:51:in `run_compiler'
** [out :: ip-address] Notice: /Exec[build_passenger]/returns: /var/lib/gems/2.1.0/gems/passenger-4.0.58/build/cplusplus_support.rb:61:in `compile_cxx'
** [out :: ip-address] Notice: /Exec[build_passenger]/returns: /var/lib/gems/2.1.0/gems/passenger-4.0.58/build/agents.rb:95:in `block in <top (required)>'
** [out :: ip-address] Notice: /Exec[build_passenger]/returns: Tasks: TOP => apache2 => buildout/agents/PassengerHelperAgent
** [out :: ip-address] Notice: /Exec[build_passenger]/returns: (See full trace by running task with --trace)
** [out :: ip-address] Error: sudo /usr/bin/ruby -S rake clean apache2 returned 1 instead of one of [0]
** [out :: ip-address] Error: /Exec[build_passenger]/returns: change from notrun to 0 failed: sudo /usr/bin/ruby -S rake clean apache2 returned 1 instead of one of [0]
** [out :: ip-address] Notice: /File[/etc/apache2/mods-available/passenger.conf]: Dependency Exec[build_passenger] has failures: true
** [out :: ip-address] Warning: /File[/etc/apache2/mods-available/passenger.conf]: Skipping because of failed dependencies
** [out :: ip-address] Notice: /File[/etc/apache2/mods-available/passenger.load]: Dependency Exec[build_passenger] has failures: true
** [out :: ip-address] Warning: /File[/etc/apache2/mods-available/passenger.load]: Skipping because of failed dependencies
** [out :: ip-address] Notice: /Exec[a2enmod passenger]: Dependency Exec[build_passenger] has failures: true
** [out :: ip-address] Warning: /Exec[a2enmod passenger]: Skipping because of failed dependencies
** [out :: ip-address] Notice: /File[/etc/apache2/sites-available/staging.conf]: Dependency Exec[build_passenger] has failures: true
** [out :: ip-address] Warning: /File[/etc/apache2/sites-available/staging.conf]: Skipping because of failed dependencies
** [out :: ip-address] Notice: /Exec[a2ensite staging]: Dependency Exec[build_passenger] has failures: true
** [out :: ip-address] Warning: /Exec[a2ensite staging]: Skipping because of failed dependencies
** [out :: ip-address] Notice: /Exec[a2dissite 000-default]: Dependency Exec[build_passenger] has failures: true
** [out :: ip-address] Warning: /Exec[a2dissite 000-default]: Skipping because of failed dependencies
** [out :: ip-address] Notice: /Service[apache2]: Dependency Exec[build_passenger] has failures: true
** [out :: ip-address] Warning: /Service[apache2]: Skipping because of failed dependencies
** [out :: ip-address] Notice: /Service[apache2]: Triggered 'refresh' from 7 events