[libmicrohttpd] Port doesn't declare its dependencies

[dg0yt]

The configuration of libmicrohttpd tests for dependencies, but the port doesn't declare any dependencies.

• curl
• gettext
• libgnutls

On macOS, it could pick up files provided either from brew or from a port if it is installed. The mixing of brew and overridden prefix creates volatile configurations.

Output from configure for macOS in CI:

configure: GNU libmicrohttpd 0.9.63 Configuration Summary:
  Target directory:  /Users/vagrant/Data/installed/x64-osx
  Cross-compiling:   no
  Operating System:  darwin20.3.0
  Shutdown of listening socket trigger select: no
  Inter-thread comm: pipe
  poll support:      yes
  epoll support:     no
  sendfile used:     yes, Darwin-style
  HTTPS support:     yes (using libgnutls)
  Threading lib:     posix
  Use thread names:  yes
  Use debug asserts: no
  Messages:          yes
  Gettext:           yes
  Basic auth.:       yes
  Digest auth.:      yes
  HTTP "Upgrade":    yes
  Postproc:          yes
  Build docs:        yes
  Build examples:    yes
  Test with libcurl: no, many unit tests will not run

(From the failed osx build in #17205. Even libcurl is found in some other contexts.)

Originally posted by @dg0yt in #17205 (comment)

[dg0yt]

• libcurl is used only for tests.
  Building tests (as well as docs, examples) shall be enabled by a separate feature.
• libgnutls is mandatory for https support, but https support is optional.
  • It is not used by the windows build system.
  • It is not found in an Azure Pipelines Ubuntu image.
  • It is found and used in an Azure Pipelines macOS image.
• gettext for translation is optional.
  • It is not used by the windows build system.
  • It is found and used in an Azure Pipelines Ubuntu image.
  • It is found and used in an Azure Pipelines macOS image.