Similar to the -std=c++0x flag for C++ code, we probe for, and add if
available, -std=c99 for C code, since the compiler may generate
different initialization code for this variant.
The reporting from the bundled CMake modules is not as polished as for
probing for C++ flags, but not bad enough to warrant rewriting the code
from scratch just to fix the output.
Put everything the user needs to link to the opm-core library into the
config-mode file, including result of probes. If this conflicts with
other packages, that is a configuration issue that must be resolved
anyway.
The advantage of doing it like this is that the user can build against
the library without knowing how it was configured; the only thing that
must be provided is the path to the config-mode module!
Users should be able to point their opm-core_DIR to the build tree of
this project and have their client programs pick up all the necessary
settings to use the library. The config mode file helps to bridge
variables from one build (of opm-core) to another (of client program).
