[BUG] linking libhttpserver might fail depending on configuration of libmicrohttpd #232

Prerequisites

• Put an X between the brackets on this line if you have checked that your issue isn't already filed: https://github.com/search?l=&q=repo%3Aetr%2Flibhttpserver&type=Issues

Description

When linking libhttpserver from an application, linker errors might pop up depending on how libmicrohttpd was configured.

We discovered this when building a custom application in a custom embedded board support package based on ptxdist using the distributed (backported) package rules for libmicrohttpd.

Steps to Reproduce

1. configure libmicrohttpd with --disable-dauth, build and install it
2. build libhttpserver against that libmicrohttpd
3. link your application against libhttpserver only, but not against libmicrohttpd

Expected behavior:

The application should not need to link libmicrohttpd by itself, because it's only a private dependency of libhttpserver.

Actual behavior:

Link errors happen, when building the application, like this:

…/sysroot-target/usr/lib/libhttpserver.so: undefined reference to `MHD_digest_auth_get_username'
…/sysroot-target/usr/lib/libhttpserver.so: undefined reference to `MHD_queue_auth_fail_response'
…/sysroot-target/usr/lib/libhttpserver.so: undefined reference to `MHD_digest_auth_check'

Reproduces how often: always

Versions

• custom embedded linux build with ptxdist 2018.05.0, kernel 4.9, gcc 4.9, glibc 2.20 (oselas toolchain 2014.12.x)
• libhttpserver version 0.18.1, compiled with custom ptxdist package
• libmicrohttpd version 0.9.66, compiled with backported package from ptxdist master

If you have problems during build:

• cross compiling toolchain
  • OSELAS.Toolchain-2014.12.2
  • arm-v5te-linux-gnueabi
  • gcc-4.9.2-glibc-2.20-binutils-2.24-kernel-3.16
• autotools version
  • autoconf-2.69
  • automake-1.15.1

Additional Information

I know it's hard to get our exact build environment, but I assume the bug description good enough to see what's going on.

Linking libhttpserver from the application works, if libmicrohttpd is configured with --enable-dauth. However if libhttpserver unconditionally uses symbols which are optional in libmicrohttpd, it should check on that.