Fix FMT_STATIC_ASSERT compile test

FMT_STATIC_ASSERT is defined in posix.h so expect_compile_error was
actually failing on a missing macro instead of the static assert.

test/compile-test/CMakeLists.txt

@@ -9,7 +9,7 @@ set(CMAKE_REQUIRED_FLAGS ${CPP11_FLAG})
 function (generate_source result fragment)
   set(${result} "
   #define FMT_HEADER_ONLY 1
-  #include \"fmt/format.h\"
+  #include \"fmt/posix.h\"
   int main() {
     ${fragment}
   }
@@ -63,6 +63,7 @@ expect_compile_error("fmt::MemoryWriter() << fmt::pad(42, 5, L' ');")
 # Formatting a wide character with a narrow format string is forbidden.
 expect_compile_error("fmt::format(\"{}\", L'a';")
 
+expect_compile("FMT_STATIC_ASSERT(true, \"this should never happen\");")
 expect_compile_error("FMT_STATIC_ASSERT(0 > 1, \"oops\");")
 
 # Make sure that compiler features detected in the header