build: Revert removal of asio find module

Explicitely prefer config mode search over find module for asio.

cmake/Findasio.cmake

@@ -0,0 +1,46 @@
+################################################################################
+# Copyright (C) 2019-2021 GSI Helmholtzzentrum fuer Schwerionenforschung GmbH  #
+#                                                                              #
+#              This software is distributed under the terms of the             #
+#              GNU Lesser General Public Licence (LGPL) version 3,             #
+#                  copied verbatim in the file "LICENSE"                       #
+################################################################################
+
+find_path(asio_INCLUDE_DIR
+  NAMES asio.hpp
+  PATH_SUFFIXES include
+)
+
+if(asio_INCLUDE_DIR)
+  find_file(asio_VERSION_HEADER "asio/version.hpp"
+    ${asio_INCLUDE_DIR}
+    NO_DEFAULT_PATH
+  )
+endif()
+
+if(asio_VERSION_HEADER)
+  file(READ "${asio_VERSION_HEADER}" _asio_VERSION_HEADER_CONTENT)
+  string(REGEX MATCH "#define ASIO_VERSION ([0-9]+)" _MATCH "${_asio_VERSION_HEADER_CONTENT}")
+  set(asio_VERSION_MACRO ${CMAKE_MATCH_1})
+  math(EXPR asio_VERSION_MAJOR "${asio_VERSION_MACRO} / 100000")
+  math(EXPR asio_VERSION_MINOR "${asio_VERSION_MACRO} / 100 % 1000")
+  math(EXPR asio_VERSION_PATCH "${asio_VERSION_MACRO} % 100")
+  set(asio_VERSION "${asio_VERSION_MAJOR}.${asio_VERSION_MINOR}.${asio_VERSION_PATCH}")
+endif()
+
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(asio
+  REQUIRED_VARS asio_INCLUDE_DIR
+  VERSION_VAR asio_VERSION
+  HANDLE_COMPONENTS
+)
+
+if(asio_FOUND AND NOT TARGET asio::asio)
+  add_library(asio::asio INTERFACE IMPORTED)
+  set_target_properties(asio::asio PROPERTIES
+    INTERFACE_COMPILE_DEFINITIONS "ASIO_STANDALONE"
+    INTERFACE_INCLUDE_DIRECTORIES "${asio_INCLUDE_DIR}"
+  )
+endif()
+