Simplify compile error tests.

test/CMakeLists.txt

@@ -3,11 +3,17 @@
 include(CheckCXXSourceCompiles)
 set(CMAKE_REQUIRED_INCLUDES ${CMAKE_CURRENT_SOURCE_DIR}/..)
 
-# Writing a wide character to a character stream Writer is forbidden.
-check_cxx_source_compiles("
+function (expect_compile_error code)
+  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 ()
+  int main() {
+    ${code}
+  }
+  " compiles)
+  if (compiles)
+    error("No compile error for: ${code}")
+  endif ()
+endfunction ()
+
+# Writing a wide character to a character stream Writer is forbidden.
+expect_compile_error("fmt::Writer() << L'a';")