Version of adding -std=c99 that works in cmake 3.2.2

[alito]

Comments refer to 9b13f2b :

The 'list(APPEND CMAKE_C_FLAGS "-std=c99")' line in pachi_py/pachi/CMakelists.txt causes cmake to emit a semi-colon before that part, breaking the build (ie

make VERBOSE=1
/usr/bin/cmake -H/usr/src/machinelearning/pachi-py/pachi_py/pachi -B/usr/src/machinelearning/pachi-py/pachi_py/build --check-build-system CMakeFiles/Makefile.cmake 0
/usr/bin/cmake -E cmake_progress_start /usr/src/machinelearning/pachi-py/pachi_py/build/CMakeFiles /usr/src/machinelearning/pachi-py/pachi_py/build/CMakeFiles/progress.marks
make -f CMakeFiles/Makefile2 all
make[1]: Entering directory '/usr/src/machinelearning/pachi-py/pachi_py/build'
make -f CMakeFiles/pachi.dir/build.make CMakeFiles/pachi.dir/depend
make[2]: Entering directory '/usr/src/machinelearning/pachi-py/pachi_py/build'
cd /usr/src/machinelearning/pachi-py/pachi_py/build && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /usr/src/machinelearning/pachi-py/pachi_py/pachi /usr/src/machinelearning$
make[2]: Leaving directory '/usr/src/machinelearning/pachi-py/pachi_py/build'
make -f CMakeFiles/pachi.dir/build.make CMakeFiles/pachi.dir/build
make[2]: Entering directory '/usr/src/machinelearning/pachi-py/pachi_py/build'
/usr/bin/cmake -E cmake_progress_report /usr/src/machinelearning/pachi-py/pachi_py/build/CMakeFiles 1
[  2%] Building C object CMakeFiles/pachi.dir/board.c.o
/usr/bin/cc   -O2 -march=native -mtune=native ;-std=c99 -O3 -DNDEBUG -I/usr/src/machinelearning/pachi-py/pachi_py/pachi    -fPIC "-D_GNU_SOURCE -Wall -Wextra -Wno-unused-functio$
cc: fatal error: no input files

Note the semi-colon in between -mtune=native and -std=c99 on the second last line. This is probably due to version-specific differences in cmake. I'm running cmake 3.2.2 (Ubuntu 15.10).

Patch changes it to code that should work on older and newer versions of cmake taken from David Grayson on stackoverflow (http://stackoverflow.com/questions/24840030/forcing-c99-in-cmake-to-use-for-loop-initial-declaration)