build: Rename DEBUG_CPPFLAGS to STRICT_CFLAGS.

build: Rename DEBUG_CPPFLAGS to STRICT_CFLAGS.

This variable is a misnomer for two reasons: First, its purpose is
unrelated to debugging. We just prefer to compile our own source
files with more strict flags than the flags used for the generated
*.cmdline.c files. Compiling those with the strict flags would
result in warnings we can not fix anyway. Second, these flags are
no preprocessor flags but compiler flags. So STRICT_CFLAGS is more
appropriate than DEBUG_CPPFLAGS.