Add facility to cross compile for Mac OS X on Linux (elastic/elasticsearch#717)

Adding a build system that makes it possible for an Ubuntu 16.04 build
server configured according to the instructions in
https://github.com/elastic/prelert-legacy/wiki/Plugin-Development-Setup-for-Mac-OS-X-cross-compiled-on-Linux
to build a Mac OS X version of the product.

Original commit: elastic/x-pack-elasticsearch@c6f39c0395

build.gradle

@@ -20,6 +20,18 @@ if (envMlAwsSecretKey != null) {
   project.ext.mlAwsSecretKey = PRELERT_AWS_SECRET_ACCESS_KEY
 }
 
+String cppCrossCompile = System.env.CPP_CROSS_COMPILE
+if (cppCrossCompile != null) {
+  project.ext.cppCrossCompile = cppCrossCompile
+} else if (project.hasProperty("CPP_CROSS_COMPILE")) {
+  project.ext.cppCrossCompile = CPP_CROSS_COMPILE
+} else {
+  project.ext.cppCrossCompile = ''
+}
+if (project.ext.cppCrossCompile != '' && project.ext.cppCrossCompile != 'macosx') {
+  throw new GradleException("CPP_CROSS_COMPILE property must be empty or 'macosx'")
+}
+
 project.ext.isWindows = OperatingSystem.current().isWindows()
 project.ext.isLinux = OperatingSystem.current().isLinux()
 project.ext.isMacOsX = OperatingSystem.current().isMacOsX()
@@ -41,7 +53,8 @@ if (['true', 'false', 'auto'].contains(cppEnabledStr) == false) {
 project.ext.cppEnabled = cppEnabledStr == 'true'
 if (cppEnabledStr == 'auto') {
   // Disable the C++ build if the 3rd party tools/libraries aren't available
-    String[] cmdArray = [ project.ext.bash, '-c', 'source cpp/set_env.sh && cpp/3rd_party/3rd_party.sh --check' ]
+  String[] cmdArray = [ project.ext.bash, '-c',
+    'export CPP_CROSS_COMPILE=' + project.ext.cppCrossCompile + ' && source cpp/set_env.sh && cpp/3rd_party/3rd_party.sh --check' ]
   Process checkProcess = Runtime.getRuntime().exec(cmdArray, null, rootDir)
   StringBuffer checkOutput = new StringBuffer()
   checkProcess.consumeProcessOutputStream(checkOutput)

elasticsearch/build.gradle

@@ -32,6 +32,8 @@ check.dependsOn noBootstrapTest
 noBootstrapTest.mustRunAfter test
 
 integTest {
+    // Cannot run integration tests when cross compiling
+    enabled project.cppCrossCompile == ''
     cluster {
         //setting 'useNativeProcess', 'true'
         distribution = 'zip'