@examples//matrix:native_matrix fails to build

[katre]

The for loops in examples/matrix/src/matrix.c don't compile because they require C99 mode:

$ bazel build @examples//matrix:native_matrix
INFO: Found 1 target...
ERROR: /home/jcater/.cache/bazel/_bazel_jcater/f161603a652c64478ce40dd2b7d0ce80/external/examples/matrix/BUILD:5:1: C++ compilation of rule '@examples//matrix:native_matrix' failed: Process exited with status 1 [sandboxed].
external/examples/matrix/src/matrix.c: In function 'matrix_transpose':
external/examples/matrix/src/matrix.c:73:3: error: 'for' loop initial declarations are only allowed in C99 mode
   for (size_t i = 0; i < q; ++i) {
   ^
external/examples/matrix/src/matrix.c:73:3: note: use option -std=c99 or -std=gnu99 to compile your code
external/examples/matrix/src/matrix.c: In function 'matrix_equal':
external/examples/matrix/src/matrix.c:107:3: error: 'for' loop initial declarations are only allowed in C99 mode
   for (size_t i = 0; i < len; ++i) {
   ^
Use --strategy=CppCompile=standalone to disable sandboxing for the failing actions.
Target @examples//matrix:native_matrix failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 0.274s, Critical Path: 0.11s