simplify concept_def.h

fbshipit-source-id: ecedf66e7d6e1ea6bcbe1ba68091f6ffdf62c260

folly/experimental/pushmi/CMakeLists.txt

@@ -21,7 +21,7 @@ target_include_directories(pushmi INTERFACE
 target_compile_features(pushmi INTERFACE cxx_std_14)
 target_compile_options(pushmi INTERFACE
     #$<$<CXX_COMPILER_ID:GNU>:-std=c++2a>
-    #$<$<CXX_COMPILER_ID:GNU>:-fconcepts>
+    $<$<CXX_COMPILER_ID:GNU>:-fconcepts>
     $<$<CXX_COMPILER_ID:GNU>:-ftemplate-backtrace-limit=0>)
 
 add_custom_target(buildSingleHeader COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_SOURCE_DIR}/buildSingleHeader.cmake WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})