Change CMAKE_C_STANDARD from 99 to 11

Set C11 as the default so that we can use static_assert().

The public headers should remain C99.

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b795715..55e3bf6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,10 @@
 
 ## [Unreleased]
 
+### Changed since 1.4.1
+
+* Require C11 for compilation. Public headers will remain C99.
+
 ## [1.4.1] - 2026-03-20
 
 ### Changed since 1.4.0

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9c83ff6..e82b969 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -47,13 +47,13 @@
 
 option(AVIF_ENABLE_NODISCARD "Add [[nodiscard]] to some functions." OFF)
 
-# Set C99 as the default
+# Set C11 as the default
 if(AVIF_ENABLE_NODISCARD)
     # [[nodiscard]] requires C23.
     set(CMAKE_C_STANDARD 23)
     set(CMAKE_C_STANDARD_REQUIRED ON)
 else()
-    set(CMAKE_C_STANDARD 99)
+    set(CMAKE_C_STANDARD 11)
 endif()
 
 # SOVERSION scheme: MAJOR.MINOR.PATCH