cmake, refactor: Rename try_add_compile_option to try_append_cflags

Actually, `try_append_cflags()` can handle a list of flags, and the new
name is similar to the one used in `configure.ac`.

cmake/TryAppendCFlags.cmake

@@ -0,0 +1,24 @@
+include(CheckCCompilerFlag)
+
+function(secp256k1_check_c_flags_internal flags output)
+  string(MAKE_C_IDENTIFIER "${flags}" result)
+  string(TOUPPER "${result}" result)
+  set(result "C_SUPPORTS_${result}")
+  if(NOT MSVC)
+    set(CMAKE_REQUIRED_FLAGS "-Werror")
+  endif()
+
+  # This avoids running a linker.
+  set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
+  check_c_compiler_flag("${flags}" ${result})
+
+  set(${output} ${${result}} PARENT_SCOPE)
+endfunction()
+
+# Append flags to the COMPILE_OPTIONS directory property if CC accepts them.
+macro(try_append_c_flags)
+  secp256k1_check_c_flags_internal("${ARGV}" result)
+  if(result)
+    add_compile_options(${ARGV})
+  endif()
+endmacro()