Resolve env variables in bazelrc

Bazel will now try to resolve environment variables
provided in bazelrc files.
If environmetn variable is not found text will remain
unchanged.

Format to be used: ${FOO}

Example:
  import ${GIT_ROOT}/some/path/bazel.rc

src/main/cpp/rc_file.cc

@@ -15,6 +15,7 @@
 #include "src/main/cpp/rc_file.h"
 
 #include <algorithm>
+#include <regex>
 #include <utility>
 
 #include "src/main/cpp/blaze_util.h"
@@ -79,6 +80,29 @@ RcFile::ParseError RcFile::ParseFile(const string& filename,
       continue;
     }
 
+    // Resolve env variables if possible
+    std::regex env_var_regex("\\$\\{(.*?)\\}");
+    std::smatch env_matcher;
+    vector <string> env_var_to_replace;
+    // Finding env variable names to replace
+    string tmp_line = line;
+    while(regex_search(tmp_line, env_matcher, env_var_regex)) {
+         env_var_to_replace.push_back(env_matcher[1].str());
+         tmp_line = env_matcher.suffix();
+    }
+    // Loop through found env variables, try to resolve them and replace if found
+    for (int i = env_var_to_replace.size() - 1; i >= 0; i--){
+      std::string env_var_p = blaze::GetEnv(env_var_to_replace[i].c_str());
+      if(!env_var_p.empty()) {
+        std::string replacement = "${"+ env_var_to_replace[i] +"}";
+        size_t pos = 0;
+        while ((pos = line.find(replacement, pos)) != std::string::npos) {
+         line.replace(pos, replacement.length(), env_var_p);
+         pos += replacement.length();
+        }
+      }
+    }
+
     vector<string> words;
 
     // This will treat "#" as a comment, and properly

src/test/cpp/rc_options_test.cc

@@ -12,6 +12,13 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#if defined(_WIN32)
+#ifndef WIN32_LEAN_AND_MEAN
+#define WIN32_LEAN_AND_MEAN
+#endif
+#include <windows.h>
+#endif
+
 #include <vector>
 
 #include "src/main/cpp/blaze_util.h"
@@ -251,6 +258,42 @@ TEST_F(RcOptionsTest, ManyArgStartup) {
       }});
 }
 
+TEST_F(RcOptionsTest, EnvVariableMissing) {
+  WriteRc("env_var_missing.bazelrc",
+          "startup ${RC_MISSING} foo");
+  SuccessfullyParseRcWithExpectedArgs(
+      "env_var_missing.bazelrc",
+      {{"startup",{"${RC_MISSING}","foo"}}});
+}
+
+TEST_F(RcOptionsTest, EnvVariableResolved) {
+#if defined(_WIN32)
+  ASSERT_NE(::SetEnvironmentVariable("RC_TEST", "startup foo"), 0);
+#else
+  setenv("RC_TEST", "startup foo", 1);
+#endif
+  WriteRc("env_var.bazelrc",
+          "${RC_TEST}");
+  SuccessfullyParseRcWithExpectedArgs(
+      "env_var.bazelrc",
+      {{"startup", {"foo"}}});
+}
+
+TEST_F(RcOptionsTest, EnvVariableResolvedMultiple) {
+#if defined(_WIN32)
+  ASSERT_NE(::SetEnvironmentVariable("RC_TEST", "startup"), 0);
+  ASSERT_NE(::SetEnvironmentVariable("RC_TEST_ARG", "foo"), 0);
+#else
+  setenv("RC_TEST", "startup", 1);
+  setenv("RC_TEST_ARG", "foo", 1);
+#endif
+  WriteRc("env_var_multi.bazelrc",
+          "${RC_TEST} ${RC_TEST_ARG}");
+  SuccessfullyParseRcWithExpectedArgs(
+      "env_var_multi.bazelrc",
+      {{"startup", {"foo"}}});
+}
+
 TEST_F(RcOptionsTest, MultipleCommands) {
   WriteRc("multiple_commands_intermixed.bazelrc",
           "startup foo\n"