You include a "PHP 4 Compatible Constructor" in your class
In PHP 5.4 with WordPress debug mode turned on, this causes an error to be generated:
Strict standards: Redefining already defined constructor for class JCP_UseGoogleLibraries in ... /wp-content/plugins/use-google-libraries/use-google-libraries.php on line 123
According to the WordPress Requirements, WordPress has required at least PHP 5.2.4 since version 3.2
I suggest that you increase your "Requires at least" to 3.2 and drop PHP 4 support - including removing your PHP 4 Compatible Constructor.