configure: Fix build when clang is installed to a non-standard prefix

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@188129 91177308-0d34-0410-b5e6-96231b3b80d8
Implement generic rint()

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@188130 91177308-0d34-0410-b5e6-96231b3b80d8
Add some missing convert_* functions

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@188131 91177308-0d34-0410-b5e6-96231b3b80d8