fix gcc compile error: use option -std=c99 or -std=gnu99 to compile your…

[taomaree]

fix gcc compile error below, add compile option -std=cc99.

src/des.c: In function 'permute':
src/des.c:188:2: error: 'for' loop initial declarations are only allowed in C99 mode
  for (size_t i = 0; i < n; i++) {
  ^
src/des.c:188:2: note: use option -std=c99 or -std=gnu99 to compile your code
make[2]: *** [CMakeFiles/gmssl.dir/build.make:588: CMakeFiles/gmssl.dir/src/des.c.o] Error 1

[Baymini-zx]

CMakeLists.txt里面增加add_definitions(-std=c99)