get rid of deprecated function

add_compiler_export_flags() is deprecated since CMake 3.0, and was
removed a long time ago in cmark via commit
abf3a7a -- but preserved in cmark-gfm's
modifications.

Author: eli-schwartz

extensions/CMakeLists.txt

@@ -24,7 +24,6 @@ include_directories(. ${CMAKE_CURRENT_BINARY_DIR})
 
 set(CMAKE_C_FLAGS_PROFILE "${CMAKE_C_FLAGS_RELEASE} -pg")
 set(CMAKE_LINKER_PROFILE "${CMAKE_LINKER_FLAGS_RELEASE} -pg")
-add_compiler_export_flags()
 
 if (CMARK_SHARED)
   add_library(${LIBRARY} SHARED ${LIBRARY_SOURCES})