* Build: further CMake updates:
- fix CLang not recognized as a gcc-compatible compiler
- support building in a separate directory (generate
  config.h in builddir/include, not sourcedir/include)
- prefer -std=gnu99 over -std=c99
* src/getopt_long.c (getopt_long): fix to match prototype
  (constify the option parameter)
better support for symbol visibility attributes:
gcc >= 4 is the official version with symbol visibility attributes,
but several vendor-supplied gcc versions, such as RedHat's gcc-3.4,
do have a backport for it. therefore, define SYM_VISIBILITY if the
cmake check for the feature is successful and adjust wildmidi_lib.h
for it.
remove alloca checks and inclusions:
alloca() doesn't seem to be used at all. therefore, remove cmake
checks and remove it from wildmidi.c.
Merge pull request #22 from sezero/master

further cmake updates/fixes and fix getopt_long()