Add a check for GCOV env variable. (#187)

* Add a check for GCOV env variable.

container/common/clang/clang.yaml.tpl

diff --git a/container/common/clang/clang.yaml.tpl b/container/common/clang/clang.yaml.tpl
index 51d97ce..8be334e 100644
--- a/container/common/clang/clang.yaml.tpl
+++ b/container/common/clang/clang.yaml.tpl
@@ -60,3 +60,5 @@
   env:
     - key: 'CC'
       value: '/usr/local/bin/clang'
+    - key: 'GCOV'
+      value: '/dev/null'

container/ubuntu14_04/layers/gcc/BUILD

diff --git a/container/ubuntu14_04/layers/gcc/BUILD b/container/ubuntu14_04/layers/gcc/BUILD
index da99ed0..4d6ab68 100644
--- a/container/ubuntu14_04/layers/gcc/BUILD
+++ b/container/ubuntu14_04/layers/gcc/BUILD
@@ -26,9 +26,6 @@
 language_tool_layer(
     name = "gcc-ltl",
     base = "@trusty//image",
-    env = {
-        "GCOV": "/dev/null",
-    },
     packages = [
         "g++",
     ],

container/ubuntu14_04/layers/gcc/gcc.yaml

diff --git a/container/ubuntu14_04/layers/gcc/gcc.yaml b/container/ubuntu14_04/layers/gcc/gcc.yaml
index bc1d68c..d8e02a0 100644
--- a/container/ubuntu14_04/layers/gcc/gcc.yaml
+++ b/container/ubuntu14_04/layers/gcc/gcc.yaml
@@ -5,3 +5,7 @@
   command: 'g++'
   args: ['--version']
   expectedOutput: ['g[+][+] [(]Ubuntu .*']
+- name: 'check-gcov'
+  command: 'gcov'
+  args: ['--version']
+  expectedOutput: ['gcov [(]Ubuntu .*']