[BOLT][CMAKE][NFC] Update runtime/CMakeLists.txt

Summary:
- Specify compiler flags for runtime libraries as BOLT_RT_FLAGS,
- Remove redundant CMake definitions.

Reviewers: maksfb

FBD34048561

bolt/runtime/CMakeLists.txt

@@ -1,9 +1,5 @@
-cmake_minimum_required(VERSION 3.1.0)
-
 include(CheckIncludeFiles)
 
-set(CMAKE_CXX_STANDARD 11)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
 project(libbolt_rt_project)
@@ -21,10 +17,16 @@ add_library(bolt_rt_hugify STATIC
   ${CMAKE_CURRENT_BINARY_DIR}/config.h
   )
 
+set(BOLT_RT_FLAGS
+  -ffreestanding
+  -fno-exceptions
+  -fno-rtti
+  -fno-stack-protector)
+
 # Don't let the compiler think it can create calls to standard libs
-target_compile_options(bolt_rt_instr PRIVATE -ffreestanding -fno-exceptions -fno-rtti -fno-stack-protector -fPIE)
+target_compile_options(bolt_rt_instr PRIVATE ${BOLT_RT_FLAGS} -fPIE)
 target_include_directories(bolt_rt_instr PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
-target_compile_options(bolt_rt_hugify PRIVATE -ffreestanding -fno-exceptions -fno-rtti -fno-stack-protector)
+target_compile_options(bolt_rt_hugify PRIVATE ${BOLT_RT_FLAGS})
 target_include_directories(bolt_rt_hugify PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
 
 install(TARGETS bolt_rt_instr DESTINATION lib)
@@ -38,9 +40,6 @@ if (CMAKE_CXX_COMPILER_ID MATCHES ".*Clang.*")
   target_include_directories(bolt_rt_instr_osx PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
   target_compile_options(bolt_rt_instr_osx PRIVATE
     -target x86_64-apple-darwin19.6.0
-    -ffreestanding
-    -fno-exceptions
-    -fno-rtti
-    -fno-stack-protector)
+    ${BOLT_RT_FLAGS})
   install(TARGETS bolt_rt_instr_osx DESTINATION lib)
 endif()