build: simplify *ifaddr handling

We really just want to skip this when building for Windows. So do that,
and remove the two header checks (we also already use both of these
headers, unguarded, in the !windows part of the codebase).

Squash the two *iffaddrs defines into one, as I haven't seen an
iffaddrs.h that implements one, but not the other.

cmake/introspection.cmake

@@ -4,7 +4,6 @@
 
 include(CheckCXXSourceCompiles)
 include(CheckCXXSymbolExists)
-include(CheckIncludeFileCXX)
 
 check_cxx_symbol_exists(O_CLOEXEC "fcntl.h" HAVE_O_CLOEXEC)
 check_cxx_symbol_exists(fdatasync "unistd.h" HAVE_FDATASYNC)
@@ -12,9 +11,7 @@ check_cxx_symbol_exists(fork "unistd.h" HAVE_DECL_FORK)
 check_cxx_symbol_exists(pipe2 "unistd.h" HAVE_DECL_PIPE2)
 check_cxx_symbol_exists(setsid "unistd.h" HAVE_DECL_SETSID)
 
-check_include_file_cxx(sys/types.h HAVE_SYS_TYPES_H)
-check_include_file_cxx(ifaddrs.h HAVE_IFADDRS_H)
-if(HAVE_SYS_TYPES_H AND HAVE_IFADDRS_H)
+if(NOT WIN32)
   include(TestAppendRequiredLibraries)
   test_append_socket_library(core_interface)
 endif()