diff --git .github/workflows/linux-build.yml .github/workflows/linux-build.yml
index 65ca764193..43c80e83aa 100644
--- .github/workflows/linux-build.yml
+++ .github/workflows/linux-build.yml
@@ -21,8 +21,8 @@ jobs:
           - "--enable-symbols=mem"
           - "--enable-symbols=all"
           - "CFLAGS=-ftrapv"
-          - "CFLAGS=-DTCL_UTF_MAX=4"
-          - "CFLAGS=-DTCL_UTF_MAX=6"
+          - "CPPFLAGS=-DTCL_UTF_MAX=4"
+          - "CPPFLAGS=-DTCL_UTF_MAX=6"
     defaults:
       run:
         shell: bash
diff --git .travis.yml .travis.yml
index 061fe2d4b0..5465fddf00 100644
--- .travis.yml
+++ .travis.yml
@@ -26,14 +26,14 @@ jobs:
       compiler: gcc
       env:
         - BUILD_DIR=unix
-        - CFGOPT=CFLAGS=-DTCL_UTF_MAX=4
+        - CFGOPT=CPPFLAGS=-DTCL_UTF_MAX=4
     - name: "Linux/GCC/Shared: UTF_MAX=6"
       os: linux
       dist: focal
       compiler: gcc
       env:
         - BUILD_DIR=unix
-        - CFGOPT=CFLAGS=-DTCL_UTF_MAX=6
+        - CFGOPT=CPPFLAGS=-DTCL_UTF_MAX=6
     - name: "Linux/GCC/Static"
       os: linux
       dist: focal
@@ -286,7 +286,7 @@ jobs:
       compiler: gcc
       env:
         - BUILD_DIR=win
-        - CFGOPT="--enable-64bit CFLAGS=-DTCL_UTF_MAX=4"
+        - CFGOPT="--enable-64bit CPPFLAGS=-DTCL_UTF_MAX=4"
       before_install: *makepreinst
     - name: "Windows/GCC/Static"
       os: windows
@@ -321,7 +321,7 @@ jobs:
       compiler: gcc
       env:
         - BUILD_DIR=win
-        - CFGOPT="CFLAGS=-DTCL_UTF_MAX=4"
+        - CFGOPT="CPPFLAGS=-DTCL_UTF_MAX=4"
       before_install: *makepreinst
     - name: "Windows/GCC-x86/Static"
       os: windows
