diff --git a/.travis.yml b/.travis.yml
index 9562d6e7..3ad04c83 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -10,12 +10,12 @@ matrix:
     # Try to emulate a C89 compiler on a POSIX system by disabling as many
     # GNU extensions as possible.
     - compiler: gcc
-      env: CFLAGS="-O2 -std=c89 -D_POSIX_C_SOURCE=200809L"
+      env: CFLAGS="-O2 -std=c89 -D_XOPEN_SOURCE=700 -Werror -Wno-error=unused-function"
     # clang with AddressSanitizer and UndefinedBehaviorSanitizer.
     - compiler: clang
       dist: trusty
       env: CONFIG="--without-python"
-           CFLAGS="-O2 -g -fno-omit-frame-pointer -fsanitize=address,undefined -fno-sanitize=float-divide-by-zero -fno-sanitize-recover=all"
+           CFLAGS="-O2 -g -fno-omit-frame-pointer -fsanitize=address,undefined -fno-sanitize=float-divide-by-zero -fno-sanitize-recover=all -Werror -Wno-error=unused-function -Wno-error=cast-align"
            UBSAN_OPTIONS=print_stacktrace=1
 script: sh autogen.sh $CONFIG && make -j2 V=1 && make check
 git: