Run travis build with gcc-8 and dpkg-buildflags

Referring to https://wiki.debian.org/Hardening :
"Several compile-time options (...) can be used to help harden a
resulting binary against memory corruption attacks.."

Debian uses a lot of hardening flags to build the packages. To catch
issues early run a build with this flags using gcc-8 in travis as well.

Author: mattthias

.travis.yml

@@ -2,6 +2,12 @@ matrix:
   include:
     - os: osx
       compiler: clang
+    - os: linux
+      compiler: gcc-8
+      env:
+        - LDFLAGS="-Wl,-z,relro"
+        - CFLAGS="-g -O2 -fstack-protector-strong -Wformat -Werror=format-security"
+        - CPPFLAGS="-Wdate-time -D_FORTIFY_SOURCE=2"
     - os: linux
       compiler: gcc
       env:
@@ -53,9 +59,12 @@ env:
 
 addons:
   apt:
+    sources:
+      - ubuntu-toolchain-r-test
     packages:
       - musl-tools
       - indent
+      - gcc-8
   homebrew:
     packages:
       - gnu-indent