abseil-cpp>=20230115.0 used by libphonenumber fails unless C++17 is used.

CMAKE_REQUIRED_FLAGS must not be passed globally as it effects all targets,
including C files.

Index: cmake/modules/FindPhonenumber.cmake
--- cmake/modules/FindPhonenumber.cmake.orig
+++ cmake/modules/FindPhonenumber.cmake
@@ -53,6 +53,7 @@ unset(optupper)
 set(CMAKE_REQUIRED_DEFINITIONS ${PHONENUMBER_DEFINITIONS})
 set(CMAKE_REQUIRED_INCLUDES ${PHONENUMBER_INCLUDE_DIRS})
 
+set(CMAKE_REQUIRED_FLAGS "-std=gnu++17")
 foreach(lib boost_thread-mt boost_thread)
 	set(CMAKE_REQUIRED_LIBRARIES ${PHONENUMBER_LDFLAGS} -l${lib})
 	CHECK_CXX_SOURCE_COMPILES("#include <phonenumbers/phonenumberutil.h>
@@ -67,6 +68,7 @@ foreach(lib boost_thread-mt boost_thread)
 		break()
 	endif(phone_number_with_${lib})
 endforeach(lib)
+unset(CMAKE_REQUIRED_FLAGS)
 
 if(NOT ENABLE_PHONENUMBER)
 	message(FATAL_ERROR "libphonenumber cannot be used. Use -DWITH_PHONENUMBER=PATH to specify the library prefix, or -DWITH_PHONENUMBER=OFF to disable it.")