OSX: update OpenSSL detection. Refs cpp-netlib#690 cpp-netlib#691

Author: anonimal

CMakeLists.txt

@@ -49,15 +49,23 @@ set(Boost_USE_MULTI_THREADED ON)
 find_package(Boost 1.58.0 REQUIRED COMPONENTS system thread)
 
 if (CPP-NETLIB_ENABLE_HTTPS)
-  if (APPLE)
-    # If we're on OS X check for Homebrew's copy of OpenSSL instead of Apple's
-    if (NOT OpenSSL_DIR)
-      execute_process (COMMAND brew --prefix openssl
-        OUTPUT_VARIABLE OPENSSL_ROOT_DIR
-        OUTPUT_STRIP_TRAILING_WHITESPACE)
-    endif()
-  endif()
-  find_package(OpenSSL)
+ if (APPLE)
+   # If we're on OS X 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 )