Fix minor clang-tidy findings

Fix issues diagnosed by clang-tidy
 - modernize-use-bool-literals
 - misc-string-integer-assignment
 - performance-faster-string-find
 - readability-redundant-string-cstr
 - readability-delete-null-pointer

Signed-off-by: Matthias Maennich <matthias@maennich.net>

Source/cmFindLibraryCommand.cxx

@@ -260,7 +260,7 @@ void cmFindLibraryHelper::RegexFromLiteral(std::string& out,
       out += "\\";
     }
 #if defined(_WIN32) || defined(__APPLE__)
-    out += tolower(ch);
+    out += static_cast<char>(tolower(ch));
 #else
     out += ch;
 #endif