Gentoo's toolchain always sets F_S (level 2 at minimum) by default.

https://bugs.gentoo.org/890072
--- a/cmake/modules/DefineCompilerFlags.cmake
+++ b/cmake/modules/DefineCompilerFlags.cmake
@@ -48,10 +48,4 @@
     if (CMAKE_BUILD_TYPE)
         string(TOLOWER "${CMAKE_BUILD_TYPE}" CMAKE_BUILD_TYPE_LOWER)
-        if (CMAKE_BUILD_TYPE_LOWER MATCHES "(release|relwithdebinfo|minsizerel)" AND (NOT ${CMAKE_C_FLAGS} MATCHES "FORTIFY_SOURCE=[3-9]"))
-            check_c_compiler_flag("-Wp,-D_FORTIFY_SOURCE=2" WITH_FORTIFY_SOURCE)
-            if (WITH_FORTIFY_SOURCE)
-                set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wp,-D_FORTIFY_SOURCE=2")
-            endif ()
-        endif()
     endif()
 endif ()
 
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -63,7 +63,3 @@
   string(TOLOWER "${CMAKE_BUILD_TYPE}" CMAKE_BUILD_TYPE_LOWER)
-  if(CMAKE_BUILD_TYPE_LOWER MATCHES "(release|relwithdebinfo|minsizerel)" AND ((NOT ${CMAKE_C_FLAGS} MATCHES "FORTIFY_SOURCE=[3-9]") AND (NOT ${CMAKE_CXX_FLAGS} MATCHES "FORTIFY_SOURCE=[3-9]")))
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_FORTIFY_SOURCE=2")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_FORTIFY_SOURCE=2")
-  endif()
 
   if (CMAKE_CXX_COMPILER MATCHES "Clang")