Use GCC 5 for compilation on Linux.

Versions before GCC 5.0 have a broken C++ 11 implementation where
std::ifstream and std::ofstream are not movable. This breaks a bunch
of places that assume they can assign it. It's fixed in GCC 5.0.

.travis.yml

@@ -10,18 +10,21 @@ matrix:
   include:
     - os: linux
       dist: trusty
-      compiler: gcc48
+      compiler: gcc5
       env:
-        - CC=gcc-4.8
-        - CXX=g++-4.8
+        - CC=gcc-5
+        - CXX=g++-5
+
+        # Travis is so parallel it runs out of memory. Limit that.
+        - NINJA_JOBS=2
       addons:
         apt:
           sources:
             - kalakris-cmake
             - ubuntu-toolchain-r-test
           packages:
-            - gcc-4.8
-            - g++-4.8
+            - gcc-5
+            - g++-5
             - libxml2-dev
             - ninja-build
             - cmake

Makefile

@@ -13,7 +13,7 @@ cmake := cmake
 cmake_flags := -G Ninja
 
 ninja := $(if $(shell which llbuild),llbuild ninja build,ninja)
-ninja_flags :=
+ninja_flags := $(if $(shell echo "$$NINJA_JOBS"),-j$(shell echo "$$NINJA_JOBS"),)
 
 all:
 	mkdir -p $(build)