cmake: don't disable _FORTIFY_SOURCE

This was inherited from libvpx where it was added long ago as an attempt
for compatibility across glibc versions. Formalizing it in the build was
never a great idea as this provides some security benefits. Projects
like Gentoo have removed this from the build flags for some time.

CFLAGS/AOM_EXTRA_C_FLAGS can still be used to set these values if
needed.

This matches the change in libvpx:
 368c5f89d configure: don't disable _FORTIFY_SOURCE

Change-Id: I7c4cee02bc75d398647d91c83e1abd494545ec58

cmake/aom_configure.cmake

diff --git a/cmake/aom_configure.cmake b/cmake/aom_configure.cmake
index 52aa37e..57417ef 100644
--- a/cmake/aom_configure.cmake
+++ b/cmake/aom_configure.cmake
@@ -422,9 +422,6 @@
     add_compiler_flag_if_supported("-Werror")
   endif()
 
-  if(build_type_lowercase MATCHES "rel")
-    add_compiler_flag_if_supported("-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0")
-  endif()
   add_compiler_flag_if_supported("-D_LARGEFILE_SOURCE")
   add_compiler_flag_if_supported("-D_FILE_OFFSET_BITS=64")