[build] set _FORTIFY_SOURCE=2 for -O* builds only

Summary:
currently building with -DCMAKE_BUILD_TYPE=Debug floods the cli
with warnings about how _FORTIFY_SOURCE=2 requires optimization.

Our Debug build compiles with -O0 because apparently -Og drops some
symbols, so this flag should only be set for Release, RelWithDebInfo
and RelMinSize

h/t @fabien for help with the CMake generator expressions

Test Plan:
  cmake .. -GNinja -DCMAKE_BUILD_TYPE=Debug
  ninja

see no warnings that _FORTIFY_SOURCE requires compiling with
optimization

  cmake .. -GNinja -DCMAKE_BUILD_TYPE=[the other three]
  ninja -nv

see the correct flags `-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2` set
in the dry run output

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6237

src/CMakeLists.txt

@@ -126,8 +126,10 @@ if(ENABLE_HARDENING)
 	# Enable stack protection
 	add_cxx_compiler_flags(-fstack-protector-all -Wstack-protector)
 
-	# Enable some buffer overflow checking
-	add_compiler_flags(-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2)
+	# Enable some buffer overflow checking, except in -O0 builds which
+	# do not support them
+	add_compiler_flags(-U_FORTIFY_SOURCE)
+	add_compile_options($<$<NOT:$<CONFIG:Debug>>:-D_FORTIFY_SOURCE=2>)
 
 	# Enable ASLR (these flags are primarily targeting MinGw)
 	add_linker_flags(-Wl,--dynamicbase -Wl,--nxcompat -Wl,--high-entropy-va)