Add GCC 4.6 to the TravisCI build.

With `-march=native`, we see assembler error messages like those described
in https://github.com/uzh-rpg/rpg_svo/issues/7 .

.travis.yml

@@ -3,6 +3,20 @@ services:
   - docker
 matrix:
   include:
+  - os: linux
+    addons:
+      apt:
+        sources:
+          - ubuntu-toolchain-r-test
+        packages:
+          - gcc-4.6
+          - g++-4.6
+    env: >-
+      TRAVIS_OS_NAME=linux
+      TRAVIS_BUILD_SYSTEM=Makefile
+      HYPERROGUE_CXX=g++-4.6
+      HYPERROGUE_USE_GLEW=1
+      HYPERROGUE_USE_PNG=1
   - os: linux
     compiler: gcc
     env: >-
@@ -126,7 +140,7 @@ script:
     ./configure CXXFLAGS="-Wall -Werror"
     make
   elif [[ "$TRAVIS_BUILD_SYSTEM" == "Makefile" ]]; then
-    make -f Makefile.simple
+    make -f Makefile.simple CXX="${HYPERROGUE_CXX-g++}"
   elif [[ "$TRAVIS_BUILD_SYSTEM" == "emscripten" ]]; then
     docker run --rm -v $(pwd):/src trzeci/emscripten make -f Makefile.simple emscripten
   else

Makefile.simple

@@ -33,7 +33,11 @@ else
   ifneq (,$(findstring clang,$(TOOLCHAIN_VERSION_S)))
     TOOLCHAIN := clang
   else
-    TOOLCHAIN := gcc
+    ifneq (,$(findstring 4.6.,$(TOOLCHAIN_VERSION_S)))
+      TOOLCHAIN := gcc46
+    else
+      TOOLCHAIN := gcc
+    endif
   endif
 endif
 
@@ -91,6 +95,13 @@ ifeq (${TOOLCHAIN},gcc)
   CXXFLAGS_EARLY += -Wno-maybe-uninitialized -Wno-missing-field-initializers -Wno-unused-parameter
 endif
 
+ifeq (${TOOLCHAIN},gcc46)
+  CXXFLAGS_EARLY += -std=c++0x
+  CXXFLAGS_EARLY += -W -Wall -Wextra -Werror
+  CXXFLAGS_EARLY += -Wno-missing-field-initializers -Wno-unused-parameter
+  CXXFLAGS_EARLY += -DUSE_STDFUNCTION=1
+endif
+
 ifeq (${TOOLCHAIN},mingw)
   CXXFLAGS_EARLY += -std=c++11 -march=native
   CXXFLAGS_EARLY += -W -Wall -Wextra -Werror