Add script to find libolm if installed without cmake support

cmake/FindOlm.cmake

+#
+# CMake module to search for the olm library
+#
+# On success, the macro sets the following variables:
+# OLM_FOUND       = if the library found
+# OLM_LIBRARY     = full path to the library
+# OLM_INCLUDE_DIR = where to find the library headers
+#
+
+find_path(OLM_INCLUDE_DIR
+          NAMES olm/olm.h
+          PATHS /usr/include
+                /usr/local/include
+                $ENV{LIB_DIR}/include
+                $ENV{LIB_DIR}/include/olm)
+
+find_library(OLM_LIBRARY
+             NAMES olm
+             PATHS /usr/lib /usr/local/lib $ENV{LIB_DIR}/lib)
+
+if(OLM_FOUND)
+  set(OLM_INCLUDE_DIRS ${OLM_INCLUDE_DIR})
+
+  if(NOT OLM_LIBRARIES)
+    set(OLM_LIBRARIES ${OLM_LIBRARY})
+  endif()
+endif()
+
+if(NOT TARGET Olm::Olm)
+  add_library(Olm::Olm UNKNOWN IMPORTED)
+  set_target_properties(Olm::Olm
+                        PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
+                                   ${OLM_INCLUDE_DIR})
+  set_property(TARGET Olm::Olm APPEND PROPERTY IMPORTED_LOCATION ${OLM_LIBRARY})
+endif()
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(OLM DEFAULT_MSG OLM_INCLUDE_DIR OLM_LIBRARY)
+
+mark_as_advanced(OLM_LIBRARY OLM_INCLUDE_DIR)