macos: fix protobuf build issue on macos 10.14.4 + xcode 10.2.1 (#2785)

CMakeLists.txt

@@ -76,7 +76,8 @@ if ("${CMAKE_GENERATOR}" MATCHES "^Visual Studio.*$")
     set(MSVS TRUE)
 endif()
 
-if (APPLE AND (NOT CMAKE_OSX_SYSROOT))
+# APPLE: Set CMAKE_OSX_SYSROOT if not set already.
+if (APPLE)
     execute_process(COMMAND sw_vers -productVersion
         OUTPUT_VARIABLE OSX_FULL_VERSION
         OUTPUT_STRIP_TRAILING_WHITESPACE)
@@ -84,17 +85,29 @@ if (APPLE AND (NOT CMAKE_OSX_SYSROOT))
     string(REGEX REPLACE "^([0-9]+\\.[0-9]+).*$" "\\1"
         OSX_SHORT_VERSION "${OSX_FULL_VERSION}")
 
-    if(NOT (OSX_SHORT_VERSION VERSION_LESS 10.14))
-        # Xcode is installed
-        set(XCODE_ISYSROOT /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX${OSX_SHORT_VERSION}.sdk)
-        message(STATUS "Trying Xcode sdk at ${XCODE_ISYSROOT}.")
-        if(NOT EXISTS ${XCODE_ISYSROOT})
+    message(STATUS "Detected: OSX ${OSX_SHORT_VERSION}")
+
+    if (CMAKE_OSX_SYSROOT)
+        message(STATUS "Using CMAKE_OSX_SYSROOT: ${CMAKE_OSX_SYSROOT}")
+    else()
+        execute_process(COMMAND xcode-select -p
+            OUTPUT_VARIABLE APPLE_DEV_DIR
+            OUTPUT_STRIP_TRAILING_WHITESPACE)
+
+        if("${APPLE_DEV_DIR}" STREQUAL "/Library/Developer/CommandLineTools")
             # Command line tools only
-            set(XCODE_ISYSROOT /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk)
+            set(XCODE_ISYSROOT ${APPLE_DEV_DIR}/SDKs/MacOSX.sdk)
             message(STATUS "Trying command line tool sdk at ${XCODE_ISYSROOT}.")
             if(NOT EXISTS ${XCODE_ISYSROOT})
                 message(FATAL_ERROR "Cannot find macos sdk.")
             endif()
+        else()
+            # Xcode is installed
+            set(XCODE_ISYSROOT ${APPLE_DEV_DIR}/Platforms/MacOSX.platform/Developer/SDKs/MacOSX${OSX_SHORT_VERSION}.sdk)
+            message(STATUS "Trying Xcode sdk at ${XCODE_ISYSROOT}.")
+            if(NOT EXISTS ${XCODE_ISYSROOT})
+                message(FATAL_ERROR "Cannot find macos sdk.")
+            endif()
         endif()
         message(STATUS "Setting CMAKE_OSX_SYSROOT for macos ${OSX_SHORT_VERSION} to ${XCODE_ISYSROOT}")
         set(CMAKE_OSX_SYSROOT ${XCODE_ISYSROOT})
@@ -102,10 +115,18 @@ if (APPLE AND (NOT CMAKE_OSX_SYSROOT))
 endif()
 
 # Setup CMAKE_ARGS to be forwarded to External Projects
-set(NGRAPH_FORWARD_CMAKE_ARGS
+if (CMAKE_OSX_SYSROOT)
+    set(NGRAPH_FORWARD_CMAKE_ARGS
+        -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
+        -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
+        -DCMAKE_OSX_SYSROOT=${CMAKE_OSX_SYSROOT}
+        )
+else()
+    set(NGRAPH_FORWARD_CMAKE_ARGS
         -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
         -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
         )
+endif()
 
 if (NOT MSVS)
     if(NOT CMAKE_BUILD_TYPE)

cmake/external_protobuf.cmake

@@ -51,6 +51,24 @@ if (WIN32)
         INSTALL_DIR "${EXTERNAL_PROJECTS_ROOT}/protobuf"
         EXCLUDE_FROM_ALL TRUE
     )
+elseif (APPLE)
+    # Don't manually set compiler on macos since it causes compile error on macos >= 10.14
+    ExternalProject_Add(
+        ext_protobuf
+        PREFIX protobuf
+        GIT_REPOSITORY ${NGRAPH_PROTOBUF_GIT_REPO_URL}
+        GIT_TAG ${NGRAPH_PROTOBUF_GIT_TAG}
+        UPDATE_COMMAND ""
+        PATCH_COMMAND ""
+        CONFIGURE_COMMAND ./autogen.sh COMMAND ./configure --prefix=${EXTERNAL_PROJECTS_ROOT}/protobuf --disable-shared CXXFLAGS=-fPIC
+        TMP_DIR "${EXTERNAL_PROJECTS_ROOT}/protobuf/tmp"
+        STAMP_DIR "${EXTERNAL_PROJECTS_ROOT}/protobuf/stamp"
+        DOWNLOAD_DIR "${EXTERNAL_PROJECTS_ROOT}/protobuf/download"
+        SOURCE_DIR "${EXTERNAL_PROJECTS_ROOT}/protobuf/src"
+        BINARY_DIR "${EXTERNAL_PROJECTS_ROOT}/protobuf/src"
+        INSTALL_DIR "${EXTERNAL_PROJECTS_ROOT}/protobuf"
+        EXCLUDE_FROM_ALL TRUE
+        )
 else()
     ExternalProject_Add(
         ext_protobuf