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-cap-list.sh

@@ -1,5 +1,5 @@
 #!/bin/sh -e
 
-cpp -dM -include linux/capability.h -include "$1" -include "$2" - </dev/null | \
+$1 -dM -include linux/capability.h -include "$2" -include "$3" - </dev/null | \
     awk '/^#define[ \t]+CAP_[A-Z_]+[ \t]+/ { print $2; }' | \
     grep -v CAP_LAST_CAP