Allow building Tenacity without Conan

Conan can still be used, but it is fully optional. This should help packagers
a lot.

Signed-off-by: Edgar <Edgar@AnotherFoxGuy.com>

cmake-proxies/cmake-modules/Findlibmp3lame.cmake

@@ -0,0 +1,76 @@
+# This file is part of Mixxx, Digital DJ'ing software.
+# Copyright (C) 2001-2020 Mixxx Development Team
+# Distributed under the GNU General Public Licence (GPL) version 2 or any later
+# later version. See the LICENSE file for details.
+
+#[=======================================================================[.rst:
+Findlibmp3lame
+-----------
+
+Finds the LAME library.
+
+Imported Targets
+^^^^^^^^^^^^^^^^
+
+This module provides the following imported targets, if found:
+
+``libmp3lame::libmp3lame``
+  The LAME library
+
+Result Variables
+^^^^^^^^^^^^^^^^
+
+This will define the following variables:
+
+``libmp3lame_FOUND``
+  True if the system has the LAME library.
+``libmp3lame_INCLUDE_DIRS``
+  Include directories needed to use LAME.
+``libmp3lame_LIBRARIES``
+  Libraries needed to link to LAME.
+
+Cache Variables
+^^^^^^^^^^^^^^^
+
+The following cache variables may also be set:
+
+``libmp3lame_INCLUDE_DIR``
+  The directory containing ``lame/lame.h``.
+``libmp3lame_LIBRARY``
+  The path to the LAME library.
+
+#]=======================================================================]
+
+find_path(libmp3lame_INCLUDE_DIR
+  NAMES lame/lame.h
+  DOC "LAME include directory")
+mark_as_advanced(libmp3lame_INCLUDE_DIR)
+
+find_library(libmp3lame_LIBRARY
+  NAMES mp3lame mp3lame-static
+  DOC "LAME library"
+)
+mark_as_advanced(libmp3lame_LIBRARY)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(
+  libmp3lame
+  DEFAULT_MSG
+  libmp3lame_LIBRARY
+  libmp3lame_INCLUDE_DIR
+)
+
+if(libmp3lame_FOUND)
+  set(libmp3lame_LIBRARIES "${libmp3lame_LIBRARY}")
+  set(libmp3lame_INCLUDE_DIRS "${libmp3lame_INCLUDE_DIR}")
+
+  if(NOT TARGET libmp3lame::libmp3lame)
+    add_library(libmp3lame::libmp3lame INTERFACE IMPORTED GLOBAL)
+    set_target_properties(libmp3lame::libmp3lame
+      PROPERTIES
+        INTERFACE_LINK_LIBRARIES "${libmp3lame_LIBRARIES}"
+        INTERFACE_INCLUDE_DIRECTORIES "${libmp3lame_INCLUDE_DIRS}"
+    )
+  endif()
+  message("Found libmp3lame: ${libmp3lame_INCLUDE_DIRS} | ${libmp3lame_LIBRARIES}")
+endif()