Silence -Wcast-qual warnings showing up with newer version of g++. (#87)

Somewhen between gcc 11 and a 14 pre-release, g++ started to complain
about the C-style casts:

subprocess.h: In function ‘int subprocess_create_ex(const char* const*, int, const char* const*, subprocess_s*)’:
subprocess.h:877:27: warning: cast from type ‘const char* const*’ to type ‘char* const*’ casts away qualifiers [-Wcast-qual]
  877 |                           (char *const *)commandLine, used_environment)) {
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~

When compiling with GCC in C-mode, the issue is still there btw. The
pragmas existing for clang are needed for GCC as well.

Co-authored-by: Harri Porten <harri.porten@qt.io>