fix cmake to make it work with FetchContent

Author: sewenew

CMakeLists.txt

@@ -145,27 +145,21 @@ include(${CMAKE_CURRENT_LIST_DIR}/cmake/FindHiredis.cmake)
 message(STATUS "redis-plus-plus check hiredis features")
 if(hiredis_FOUND)
     set(HIREDIS_FEATURE_TEST_INCLUDE ${hiredis_INCLUDE_DIRS})
-    set(HIREDIS_FEATURE_TEST_LIB ${REDIS_PLUS_PLUS_HIREDIS_LIBS})
 else()
     set(HIREDIS_FEATURE_TEST_INCLUDE ${HIREDIS_HEADER})
-    set(HIREDIS_FEATURE_TEST_LIB ${HIREDIS_LIB})
 endif()
 set(HIREDIS_FEATURE_TEST_HEADER "${HIREDIS_FEATURE_TEST_INCLUDE}/hiredis/hiredis.h")
 
-include(CheckSymbolExists)
+file(READ "${HIREDIS_FEATURE_TEST_HEADER}" HIREDIS_HEADER_CONTENT)
 
-# Add hiredis to CMAKE_REQUIRED_LIBRARIES
-set(CMAKE_REQUIRED_LIBRARIES_BACK ${CMAKE_REQUIRED_LIBRARIES})
-set(CMAKE_REQUIRED_LIBRARIES ${HIREDIS_FEATURE_TEST_LIB})
-
-CHECK_SYMBOL_EXISTS(redisEnableKeepAliveWithInterval ${HIREDIS_FEATURE_TEST_HEADER} REDIS_PLUS_PLUS_HAS_redisEnableKeepAliveWithInterval)
+string(FIND "${HIREDIS_HEADER_CONTENT}" redisEnableKeepAliveWithInterval redisEnableKeepAliveWithInterval_POS)
+if(${redisEnableKeepAliveWithInterval_POS} GREATER -1)
+    set(REDIS_PLUS_PLUS_HAS_redisEnableKeepAliveWithInterval ON)
+endif()
 
 set(REDIS_PLUS_PLUS_GENERATED_HEADER_DIR ${CMAKE_CURRENT_BINARY_DIR}/${REDIS_PLUS_PLUS_HEADER_DIR})
 configure_file(${CMAKE_CURRENT_SOURCE_DIR}/hiredis_features.h.in ${CMAKE_CURRENT_BINARY_DIR}/${REDIS_PLUS_PLUS_SOURCE_DIR}/hiredis_features.h)
 
-# Restore CMAKE_REQUIRED_LIBRARIES
-set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES_BACK})
-
 # Build static library
 option(REDIS_PLUS_PLUS_BUILD_STATIC "Build static library" ON)
 message(STATUS "redis-plus-plus build static library: ${REDIS_PLUS_PLUS_BUILD_STATIC}")