CMake: Only use -Wpedantic if supported by the compiler
CMake: Only use -Wpedantic if supported by the compiler

Older versions of GCC (GCC 4.7) found e.g. in Debian oldstable don't
support that flag.

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio