Cross compile nnn on Travis CI

.travis.yml

@@ -11,6 +11,16 @@ matrix:
     - os: linux
       dist: trusty
       compiler: clang
+    - os: linux
+      compiler: arm-linux-gnueabihf-gcc
+      addons:
+          apt:
+              packages: ['gcc-arm-linux-gnueabihf', 'libc6-dev-armhf-cross']
+    - os: linux
+      compiler: aarch64-linux-gnu-gcc
+      addons:
+          apt:
+              packages: ['gcc-aarch64-linux-gnu', 'libc6-dev-arm64-cross']
     - os: osx
       compiler: gcc
     - os: osx