I was trying to install passenger-install-apache2-module on my Debian box, and i got this error message:

Checking for required software...
 * GNU C++ compiler... found at /usr/bin/g++
 * Curl development headers with SSL support... not found
 * OpenSSL development headers... found
 * Zlib development headers... found
 * 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/apache2
 * Apache 2 development headers... found at /usr/bin/apxs2
 * Apache Portable Runtime (APR) development headers... found at /usr/bin/apr-1-config
 * Apache Portable Runtime Utility (APU) development headers... found at /usr/bin/apu-1-config

Some required software is not installed.

The OS is missing libcurl4-openssl-dev, so let’s install it:

 sudo apt-get install libcurl4-openssl-dev

Now it works !