correct extconf to respect CXX flag

.travis.yml

@@ -1,6 +1,6 @@
 language: ruby
 env:
-  - CXX=g++-4.8 CC=gcc-4.8
+  - CXX=g++-4.8
 rvm:
   - 2.0
   - 2.1
@@ -8,13 +8,10 @@ rvm:
   - 2.3.1
 before_install:
   - gem install bundler -v 1.12.0
-  - update-alternatives --auto g++
-  - update-alternatives --auto gcc
 cache: bundler
 addons:
   apt:
     sources:
       - ubuntu-toolchain-r-test
     packages:
-      - gcc-4.8
       - g++-4.8

ext/mini_racer_extension/extconf.rb

@@ -10,6 +10,11 @@ $CPPFLAGS += " -fPIC" unless $CPPFLAGS.split.include? "-rdynamic" or RUBY_PLATFO
 $CPPFLAGS += " -std=c++0x"
 $CPPFLAGS += " -fpermissive"
 
+if ENV['CXX']
+  puts "SETTING CXX"
+  CONFIG['CXX'] = ENV['CXX']
+end
+
 CONFIG['LDSHARED'] = '$(CXX) -shared' unless RUBY_PLATFORM =~ /darwin/
 if CONFIG['warnflags']
   CONFIG['warnflags'].gsub!('-Wdeclaration-after-statement', '')