WASHINGTON — President Barack Obama signed a $24 billion economic stimulus bill into law Friday, giving tax incentives to prospective homebuyers and additional jobless benefits to those idled by the business slump. The bill-signing came a day after the House, displaying rare bipartisan agreement over the troubling employment picture nationally, voted 403-12 to pass the measure. The Senate had approved it unanimously on Wednesday. The White House said that the law, which also includes tax cuts for struggling businesses, builds on provisions in the $787 billion stimulus package enacted last February to avert an economic meltdown. ‘‘The need for such a measure was made clear by the jobs report that we received this morning,’’ Obama said, citing Friday’s government report the jobless rate hit 10.2 percent last month, the highest since 1983.