Fix transform without a lambda

Catch apparently supports targeting C++03, so use an inline function
instead.

include/internal/catch_common.hpp

@@ -21,9 +21,11 @@ namespace Catch {
     bool contains( std::string const& s, std::string const& infix ) {
         return s.find( infix ) != std::string::npos;
     }
+    char toLowerCh(char c) {
+        return static_cast<char>( ::tolower( c ) );
+    }
     void toLowerInPlace( std::string& s ) {
-        std::transform( s.begin(), s.end(), s.begin(),
-            [](char c) { return static_cast<char>(::tolower(c)); } );
+        std::transform( s.begin(), s.end(), s.begin(), toLowerCh );
     }
     std::string toLower( std::string const& s ) {
         std::string lc = s;