It's case-insensitive by default.
From the Force.com SOQL and SOSL Reference
on WHERE conditionExpression:
comparisonOperator Case-insensitive operators that compare values.
Operator =
Name Equals
Description Expression is true if the value in the specified fieldName equals the specified value in the expression. String comparisons using the equals operator are case-sensitive for unique case-sensitive fields and case-insensitive for all other fields.