ENABLE_IPV6 depends on HAVE_GETADDRINFO or you will get a
Curl_getaddrinfo_ex error. Enable IPv6 by default, disabling it if
struct sockaddr_in6 is not found in netinet/in.h.

Note that HAVE_GETADDRINFO_THREADSAFE is still not set as it needs more
platform checks even though POSIX requires a thread-safe getaddrinfo.

The cache hack is needed because ENABLE_IPV6 is a macro guard. HAVE_IPV6
would be a nicer name, but is a more evasive change.

Verified on Arch Linux x86_64 with glibc 2.20-2 and Linux 3.16-rc7.

Signed-off-by: Peter Wu <peter_at_lekensteyn.nl>

---
 CMakeLists.txt     | 16 +++++++++++++++-
 docs/INSTALL.cmake |  1 -
 2 files changed, 15 insertions(+), 2 deletions(-)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 13bd39e..7ac69fc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -171,8 +171,21 @@ option(CURL_DISABLE_VERBOSE_STRINGS "to disable verbose strings" OFF)
 mark_as_advanced(CURL_DISABLE_VERBOSE_STRINGS)
 option(DISABLED_THREADSAFE "Set to explicitly specify we don't want to use thread-safe functions" OFF)
 mark_as_advanced(DISABLED_THREADSAFE)
-option(ENABLE_IPV6 "Define if you want to enable IPv6 support" OFF)
+option(ENABLE_IPV6 "Define if you want to enable IPv6 support" ON)
 mark_as_advanced(ENABLE_IPV6)
+if(ENABLE_IPV6)
+  include(CheckStructHasMember)
+  check_struct_has_member("struct sockaddr_in6" sin6_addr "netinet/in.h"
+                          HAVE_SOCKADDR_IN6_SIN6_ADDR)
+  check_struct_has_member("struct sockaddr_in6" sin6_scope_id "netinet/in.h"
+                          HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID)
+  if(NOT HAVE_SOCKADDR_IN6_SIN6_ADDR)
+    message(WARNING "struct sockaddr_in6 not available, disabling IPv6 support")
+    # Force the feature off as this name is used as guard macro...
+    set(ENABLE_IPV6 OFF
+        CACHE BOOL "Define if you want to enable IPv6 support" FORCE)
+  endif()
+endif()
 
 option(ENABLE_MANUAL "to provide the built-in manual" ON)
 unset(USE_MANUAL CACHE) # TODO: cache NROFF/NROFF_MANOPT/USE_MANUAL vars?
@@ -731,6 +744,7 @@ check_symbol_exists(strerror_r     "${CURL_INCLUDES}" HAVE_STRERROR_R)
 check_symbol_exists(siginterrupt   "${CURL_INCLUDES}" HAVE_SIGINTERRUPT)
 check_symbol_exists(perror         "${CURL_INCLUDES}" HAVE_PERROR)
 check_symbol_exists(fork           "${CURL_INCLUDES}" HAVE_FORK)
+check_symbol_exists(getaddrinfo    "${CURL_INCLUDES}" HAVE_GETADDRINFO)
 check_symbol_exists(freeaddrinfo   "${CURL_INCLUDES}" HAVE_FREEADDRINFO)
 check_symbol_exists(freeifaddrs    "${CURL_INCLUDES}" HAVE_FREEIFADDRS)
 check_symbol_exists(pipe           "${CURL_INCLUDES}" HAVE_PIPE)
diff --git a/docs/INSTALL.cmake b/docs/INSTALL.cmake
index dd8c936..b2924e6 100644
--- a/docs/INSTALL.cmake
+++ b/docs/INSTALL.cmake
@@ -31,7 +31,6 @@ Current flaws in the curl CMake build
    - Doesn't allow build curl and libcurl debug enabled
    - Doesn't allow a custom CA bundle path
    - Doesn't allow you to disable specific protocols from the build
-   - Doesn't properly enable IPv6 support by default
    - Doesn't find or use krb4 or GSS
    - Rebuilds test files too eagerly, but still can't run the tests
 
-- 
2.1.1
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