[CMake] Remove spurious character in C++14 test

Probably introduced accidentally in commit daa2e9f4.

cmake/modules/CheckCXXFeatures.cmake

@@ -60,7 +60,7 @@ string(REPLACE ";" "\;" cxx_14_test
 
   int main() {
     // lambdas with auto parameters are C++14 - so this checks the compiler
-    auto l = [](auto x) { return x; };a
+    auto l = [](auto x) { return x; };
     // std::make_unique() is a C++14 library feature - this checks whether the
     // compiler uses a C++14 compliant library.
     auto v = std::make_unique<int>(l(0));