build: fix symbol and library detection

Do not unset CMAKE_REQUIRED_* variables in order to allow various
feature tests to work properly with nonstandard locations.

Fixes: #186
Signed-off-by: Jo-Philipp Wich <[email protected]>

CMakeLists.txt

@@ -103,8 +103,8 @@ if(NOT FMOD_FUNCTION_EXISTS)
   target_link_libraries(libucode m)
 endif()
 
-set(CMAKE_REQUIRED_INCLUDES ${JSONC_INCLUDE_DIRS})
-set(CMAKE_REQUIRED_LIBRARIES ${JSONC_LINK_LIBRARIES})
+list(APPEND CMAKE_REQUIRED_INCLUDES ${JSONC_INCLUDE_DIRS})
+list(APPEND CMAKE_REQUIRED_LIBRARIES ${JSONC_LINK_LIBRARIES})
 check_symbol_exists(json_tokener_get_parse_end "json-c/json.h" HAVE_PARSE_END)
 if(HAVE_PARSE_END)
   add_definitions(-DHAVE_PARSE_END)
@@ -117,8 +117,6 @@ check_symbol_exists(json_object_new_uint64 "json-c/json.h" HAVE_JSON_UINT64)
 if(HAVE_JSON_UINT64)
   add_definitions(-DHAVE_JSON_UINT64)
 endif()
-unset(CMAKE_REQUIRED_INCLUDES)
-unset(CMAKE_REQUIRED_LIBRARIES)
 
 set(LIBRARIES "")
 
@@ -237,11 +235,10 @@ if(ULOOP_SUPPORT)
   add_library(uloop_lib MODULE lib/uloop.c)
   set_target_properties(uloop_lib PROPERTIES OUTPUT_NAME uloop PREFIX "")
   target_link_options(uloop_lib PRIVATE ${UCODE_MODULE_LINK_OPTIONS})
-  set(CMAKE_REQUIRED_LIBRARIES ${libubox})
+  list(APPEND CMAKE_REQUIRED_LIBRARIES ${libubox})
   check_function_exists(uloop_timeout_remaining64 REMAINING64_FUNCTION_EXISTS)
   check_function_exists(uloop_interval_set INTERVAL_FUNCTION_EXISTS)
   check_function_exists(uloop_signal_add SIGNAL_FUNCTION_EXISTS)
-  unset(CMAKE_REQUIRED_LIBRARIES)
   if(REMAINING64_FUNCTION_EXISTS)
     target_compile_definitions(uloop_lib PUBLIC HAVE_ULOOP_TIMEOUT_REMAINING64)
   endif()