Build: update Ecasound detection to use libecasoundc-config script

[alios]

Problem

ecasound is configured by search a given list of prefix for essential files to derive CPPFLAGS, LIBS and INCLUDE_DIR as there is no pkg-config support for it.

A comment from configure.ac deocuments this

dnl Checking for libecasoundc. It does not provide pkg-config and installs
dnl on some systems to non-standard path /usr/include/libecasoundc

But on systems where things are installed in non default prefixes (f.e. nixos where everyhing is stored under /nix) this does not work.

Sollution

ecasound comes with a tool libecasoundc-config which beaves exactly like pkg-config but as its own tool and not as a .pc file.

This PR changes the configure.ac and replaces the scripted search by utilizing that tool.

Note

Have only tested this under linux x64 on nixos.

[alios]

The CI fails, but it looks like it is unrelated to my patch. It fails in the Step "Installing apt packages", with:

/bin/bash: line 1: apt-key: command not found
curl: (23) Failure writing output to destination, passed 1369 returned 1367

Exited with code exit status 127

This is also mentioned here: #413 (comment)

[mgeier]

Thanks for this PR, and sorry for my late response!

I have fixed the CI failures, can you please rebase your PR?

[mgeier]

Are those checks still necessary? Wouldn't libecasoundc-config fail if those files aren't there?

Suggested change

	AC_MSG_CHECKING([for ecasoundc.h via libecasoundc-config])
	AC_CHECK_HEADER([ecasoundc.h], [have_ecasound=yes], [have_ecasound=no])
	AC_MSG_CHECKING([for eca-control-interface.h via libecasoundc-config])
	AC_CHECK_HEADER([eca-control-interface.h], , [have_ecasound=no])
	AC_MSG_CHECKING([for Ecasound library via libecasoundc-config])
	AC_SEARCH_LIBS([eci_init], [ecasoundc], , [have_ecasound=no])
	AC_MSG_RESULT([$have_ecasound])