Remove "-rdynamic" flag for static builds (commonmark#300)

cmake adds `-rdynamic` on linux, even for static builds.
This commit removes it for linux builds by resetting
`CMAKE_SHARED_LIBRARY_LINK_C_FLAGS`.

CMakeLists.txt

@@ -12,6 +12,13 @@ option(CMARK_STATIC "Build static libcmark library" ON)
 option(CMARK_SHARED "Build shared libcmark library" ON)
 option(CMARK_LIB_FUZZER "Build libFuzzer fuzzing harness" OFF)
 
+# The Linux modules distributed with CMake add "-rdynamic" to the build flags
+# which is incompatible with static linking under certain configurations.
+# Unsetting CMAKE_SHARED_LIBRARY_LINK_C_FLAGS ensures this does not happen.
+if(CMARK_STATIC AND "${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
+  SET(CMAKE_SHARED_LIBRARY_LINK_C_FLAGS)
+endif()
+
 add_subdirectory(src)
 if(CMARK_TESTS AND (CMARK_SHARED OR CMARK_STATIC))
   add_subdirectory(api_test)