CMake: Find system libweb or dynstr if available

So far, slcl's build system would always build libweb and dynstr.
However, this is discouraged by distribution packagers.

CMakeLists.txt

@@ -12,7 +12,17 @@ add_executable(${PROJECT_NAME}
 )
 target_compile_options(${PROJECT_NAME} PRIVATE -Wall)
 target_compile_definitions(${PROJECT_NAME} PRIVATE _FILE_OFFSET_BITS=64)
-add_subdirectory(libweb)
+set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${CMAKE_CURRENT_LIST_DIR}/cmake)
+find_package(web 0.1.0)
+
+if(WEB_FOUND)
+    find_package(dynstr 0.1.0)
+else()
+    message(STATUS "Using in-tree libweb")
+    add_subdirectory(libweb)
+    #dynstr is already provided by libweb.
+endif()
+
 find_package(cJSON 1.0 REQUIRED)
 find_package(OpenSSL 2.0 REQUIRED)
 target_link_libraries(${PROJECT_NAME} PRIVATE web dynstr cjson OpenSSL::SSL)

cmake/Findweb.cmake

@@ -0,0 +1,24 @@
+mark_as_advanced(WEB_LIBRARY WEB_INCLUDE_DIR)
+find_library(WEB_LIBRARY NAMES libweb web)
+
+find_path(WEB_INCLUDE_DIR
+    NAMES
+        handler.h
+        html.h
+        http.h
+        server.h
+        wildcard_cmp.h
+    PATH_SUFFIXES libweb include/libweb)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(web
+    DEFAULT_MSG WEB_LIBRARY WEB_INCLUDE_DIR)
+
+if(WEB_FOUND)
+    if(NOT TARGET web)
+        add_library(web UNKNOWN IMPORTED)
+        set_target_properties(web PROPERTIES
+            INTERFACE_INCLUDE_DIRECTORIES "${WEB_INCLUDE_DIR}"
+            IMPORTED_LOCATION "${WEB_LIBRARY}")
+    endif()
+endif()