ci: enable format attributes in ncurses headers

Avoid format string issues like bfcb8ca0 by helping compilers spot such
bogus usages.

Also use LTO and O3 in the full-featured gcc job, which might trigger
additional warnings on advanced inlining, like
3695cbd5d8 and
ad3acfc847

.github/workflows/ci.yml

@@ -2,6 +2,11 @@ name: CI
 
 on: [ push, pull_request ]
 
+env:
+  # Enable format attributes in ncurses headers
+  # Enable fortified memory/string handling
+  CPPFLAGS: -DGCC_PRINTF -DGCC_SCANF -D_FORTIFY_SOURCE=2
+
 jobs:
   build-ubuntu-latest-minimal-gcc:
     runs-on: ubuntu-latest
@@ -47,6 +52,10 @@ jobs:
 
   build-ubuntu-latest-full-featured-gcc:
     runs-on: ubuntu-latest
+    # Enable LTO, might trigger additional warnings on advanced inlining
+    env:
+      CFLAGS: -O3 -g -flto
+      LDFLAGS: -O3 -g -flto
     steps:
     - uses: actions/checkout@v2
     - name: Install Dependencies