OSX: update OpenSSL detection. Closes cpp-netlib#690

- Cherry-pick's acb020b 0.13-release
- References cpp-netlib#691

Author: anonimal

CMakeLists.txt

@@ -48,7 +48,23 @@ set(Boost_USE_MULTI_THREADED ON)
 find_package(Boost 1.58.0 REQUIRED COMPONENTS system)
 
 if (CPP-NETLIB_ENABLE_HTTPS)
-  find_package( OpenSSL )
+  if (APPLE)
+    # If we're on OSX check for Homebrew's copy of OpenSSL instead of Apple's
+    if (NOT OpenSSL_DIR)
+      find_program(HOMEBREW brew)
+      if (HOMEBREW STREQUAL "HOMEBREW-NOTFOUND")
+        message(WARNING "Homebrew not found: not using Homebrew's OpenSSL")
+        if (NOT OPENSSL_ROOT_DIR)
+          message(WARNING "Use -DOPENSSL_ROOT_DIR for non-Apple OpenSSL")
+        endif()
+      else()
+        execute_process(COMMAND brew --prefix openssl
+          OUTPUT_VARIABLE OPENSSL_ROOT_DIR
+          OUTPUT_STRIP_TRAILING_WHITESPACE)
+      endif()
+    endif()
+  endif()
+  find_package(OpenSSL)
 endif()
 
 find_package( Threads )