BUG #15414: extension compilation fails when c++ files are named with.cc

The following bug has been logged on the website:

Bug reference:      15414
Logged by:          Jerry Sievert
Email address:      code@legitimatesounding.com
PostgreSQL version: 11beta4
Operating system:   any
Description:

when --with-llvm is configured, compiling c++ extensions that use .cc and
not .cpp fail with no rule to create .bc files.

.cc is a fairly standard c++ file ending, and has been in use by a popular
pg extension for 7 years.  pg should be able to understand this file ending,
and not fail when trying to compile them against pg11 with llvm configured.