meson: make cpp invocations cross-compilation friendly

This implementation assumes that the arguments in compiler.cmd_array()
don't contain any spaces. Since we are only interested in compilation
on Linux, I think this is a safe assumption.

Solution suggested by Nirbheek Chauhan.

src/basic/generate-af-list.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
 
-cpp -dM -include sys/socket.h - </dev/null | \
+$1 -E -dM -include sys/socket.h - </dev/null | \
     grep -Ev 'AF_UNSPEC|AF_MAX' | \
     awk '/^#define[ \t]+AF_[^ \t]+[ \t]+PF_[^ \t]/ { print $2; }'