changed C standard for C compiler

[sircodalotkob]

The common code of img4dc uses the C function realpath() to canonicalize/cleanup paths. Since -std=c99 was added to CMakeLists.txt, the code couldn't be built on Linux anymore, because realpath() is not part of C99. It is, however, part of the GNU extensions to the C99 standard, so using -std=gnu99 does the trick.