demmt: Use POSIX basename(3)
When building on FreeBSD 14, the compiler can not find
the prototype of basename(3):
[ 91%] Building C object demmt/CMakeFiles/demmt.dir/config.c.o
/home/saper/src/envytools/demmt/config.c:508:24: warning: implicit declaration of function 'basename' is invalid in C99 [-Wimplicit-function-declaration]
const char *base = basename(filename);
^
/home/saper/src/envytools/demmt/config.c:508:17: warning: incompatible integer to pointer conversion initializing 'const char *' with an expression of type 'int' [-Wint-conversion]
const char *base = basename(filename);
^ ~~~~~~~~~~~~~~~~~~
2 warnings generated.
Reduce dependency on GNU libc behaviour and use POSIX basename(3)
on all platforms.