Don't clobber our toolchain defaults.

https://bugs.gentoo.org/895260
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -100,7 +100,6 @@ set(flags_to_test
     -Wformat-security
     -fPIE
     -fPIC
-    -D_FORTIFY_SOURCE=2
     /GS
     /sdl)
 if(MSVC)
--- a/gstreamer-plugin/CMakeLists.txt
+++ b/gstreamer-plugin/CMakeLists.txt
@@ -25,8 +25,7 @@ set(flags_to_test
     -fstack-protector-strong
     -fPIE
     -fPIC
-    -flto
-    -D_FORTIFY_SOURCE=2)
+    -flto)
 if(NOT MSVC)
     list(INSERT flags_to_test 0 -Wall)
 else()
--- a/gstreamer-plugin/meson.build
+++ b/gstreamer-plugin/meson.build
@@ -22,7 +22,6 @@ svthevcenc_dep = dependency('SvtHevcEnc')
 cc = meson.get_compiler('c')
 cc_flags = [
     '-O2',
-    '-D_FORTIFY_SOURCE=2',
     '-Wformat',
     '-Wformat-security',
     '-fPIE',