fix: cmake windows xp build

[matthiasng]

The following line always evaluates to true if you compile cmake on a system with inet_pton. This lead to no way to build curl on a newer system for Windows XP.

check_symbol_exists(inet_pton      "${CURL_INCLUDES}" HAVE_INET_PTON)

inet_pton is in WS2tcpip.h enclosed by #if (_WIN32_WINNT >= 0x0600). So we have to set CMAKE_REQUIRED_DEFINITIONS that check_symbol_exists(inet_pton ...) uses the right define.

[MarcelRaad]

Thanks, makes sense!

I'm now trying to find out why it worked for me when testing be578ee. I assumed that check_symbol_exists respects COMPILE_DEFINITIONS. I'm pretty sure I tested both post and pre Windows Vista cases with Visual Studio as well as MinGW. 😕

[matthiasng]

Maybe you stumbled over another little bug i encountered yesterday.

CMake only runs check_symbol_exists at configuration time.
This results in the following behaviour:

1. First run (without variables)
   • check_symbol_exists will set HAVE_INET_PTON to 1
2. Change ENABLE_INET_PTON or CURL_TARGET_WINDOWS_VERSION and rerun cmake
   • HAVE_INET_PTON stays 1 (cached)

Maybe this can be solve with using a temporary variable for check_symbol_exists and an additional if to set HAVE_INET_PTON depending on the result of check_symbol_exists, ENABLE_INET_PTON and CURL_TARGET_WINDOWS_VERSION.

[bagder]

Thanks!