gerbera: Always use llvm runtime with clang
gerbera: Always use llvm runtime with clang

It expects libc++ with clang
Fixes

| CMake Error at CMakeLists.txt:316 (message):
|   gerbera requires C++17 Optional to be available.  Check your compiler is
|   >=GCC 7.1 or >=libc++ 5

Signed-off-by: Khem Raj <729d64b6f67515e258459a5f6d20ec88b2caf8df@gmail.com>

kraj/meta-clang,kraj/meta-clang,kraj/meta-clang