Add compile error tests.

test/CMakeLists.txt

@@ -0,0 +1,12 @@
+# Test if compile errors are produced where necessary.
+
+include(CheckCXXSourceCompiles)
+set(CMAKE_REQUIRED_INCLUDES ${CMAKE_CURRENT_SOURCE_DIR}/..)
+
+check_cxx_source_compiles("
+  #include \"format.cc\"
+  int main() { fmt::Writer() << L'a'; }
+  " WRITE_WCHAR_COMPILES)
+if (WRITE_WCHAR_COMPILES)
+  error("No compile error for: fmt::Writer() << L'a'")
+endif ()