I did this

Attempted to build libcurl against libressl by specifying a path thusly:

./configure --with-ssl=/path/to/libressl ...

However, I neglected to build libressl prior to doing this hence /path/to/libressl didn't exist.

I expected the following

An error telling me I gave a nonexistent path to --with-ssl.

Ideally if the directory exists but doesn't contain openssl headers/libraries this should also be an error but maybe this is asking too much.

Actual result

libcurl built itself against system openssl which is an entirely different ssl implementation without any errors.

Further background

There are two mentions of /path/to/libressl in configure output:

configure: PKG_CONFIG_LIBDIR will be set to "/path/to/libressl/lib/pkgconfig"
configure: Added /path/to/libressl/lib to CURL_LIBRARY_PATH

So, my specified path is simply added (prepended I imagine) to search directories.

curl/libcurl version

7.60.0 from source/release tarball

operating system

Ubuntu Trusty VM

Note

This issue may well affect other --with-* options, I added a generic guard for directory existence to my build process now.