Clean up setting up GC sections

Summary:
1. Use target_link_options instead of supplying the linker argument as
a dependency
2. Enable GC sections for executables
3. Enable ffunction-sections and fdata-sections
4. Change the setup so it is easier to include other linker flags (like
ICF in the next diff)

Reviewed By: jpporto

Differential Revision: D35347443

fbshipit-source-id: 2c255e584d30f95504c8c9c0142f44561c47be3f

API/hermes/CMakeLists.txt

@@ -58,9 +58,8 @@ endif()
 target_link_libraries(libhermes
   jsi
   ${LIBHERMES_VM_DEP}
-  # Linker flags
-  ${OPTIONAL_GC_SECTIONS}
 )
+target_link_options(libhermes PRIVATE ${HERMES_EXTRA_LINKER_FLAGS})
 
 # Export the required header directory
 target_include_directories(libhermes PUBLIC ..)

CMakeLists.txt

@@ -3,7 +3,7 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
-cmake_minimum_required(VERSION 3.8.0)
+cmake_minimum_required(VERSION 3.13.0)
 
 # Set the VERSION variables based on the project command
 if (POLICY CMP0048)
@@ -301,9 +301,9 @@ CHECK_CXX_COMPILER_FLAG("" HAVE_GC_SECTIONS)
 set(CMAKE_EXE_LINKER_FLAGS "${OLD_CMAKE_EXE_LINKER_FLAGS}")
 
 if(HAVE_GC_SECTIONS)
-  set(OPTIONAL_GC_SECTIONS "-Wl,--gc-sections")
-else()
-  set(OPTIONAL_GC_SECTIONS "")
+  add_flag_if_supported("-ffunction-sections" FUNCTION_SECTIONS)
+  add_flag_if_supported("-fdata-sections" DATA_SECTIONS)
+  list(APPEND HERMES_EXTRA_LINKER_FLAGS "LINKER:--gc-sections")
 endif()
 
 # Make the HERMES_RELEASE_VERSION accessible for version printing in C++.

cmake/modules/Hermes.cmake

@@ -133,6 +133,7 @@ function(add_hermes_executable name)
   cmake_parse_arguments(ARG "" "" "LINK_LIBS" ${ARGN})
   add_executable(${name} ${ARG_UNPARSED_ARGUMENTS})
   target_link_libraries(${name} ${ARG_LINK_LIBS} ${HERMES_LINK_COMPONENTS})
+  target_link_options(${name} PRIVATE ${HERMES_EXTRA_LINKER_FLAGS})
   hermes_update_compile_flags(${name})
 endfunction(add_hermes_executable)