Make enabled warnings errors during CI builds

Related to #1043.

.appveyor.yml

@@ -4,6 +4,7 @@ branches:
         - master
 skip_tags: true
 environment:
+    CFLAGS: /WX
     matrix:
         - BUILD_SHARED_LIBS: ON
         - BUILD_SHARED_LIBS: OFF

.travis.yml

@@ -20,8 +20,11 @@ addons:
             - libxcursor-dev
             - libxi-dev
 env:
-    - BUILD_SHARED_LIBS=ON
-    - BUILD_SHARED_LIBS=OFF
+    global:
+        - CFLAGS=-Werror
+    matrix:
+        - BUILD_SHARED_LIBS=ON
+        - BUILD_SHARED_LIBS=OFF
 script:
     - mkdir build
     - cd build