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/udev/generate-keyboard-keys-list.sh

@@ -1,4 +1,4 @@
 #!/bin/sh -e
 
-cpp -dM -include linux/input.h - </dev/null | \
+$1 -dM -include linux/input.h - </dev/null | \
     awk '/^#define[ \t]+KEY_[^ ]+[ \t]+[0-9K]/ { if ($2 != "KEY_MAX") { print $2 } }'

src/udev/meson.build

@@ -58,7 +58,7 @@ generate_keyboard_keys_list = find_program('generate-keyboard-keys-list.sh')
 keyboard_keys_list_txt = custom_target(
     'keyboard-keys-list.txt',
     output : 'keyboard-keys-list.txt',
-    command : [generate_keyboard_keys_list],
+    command : [generate_keyboard_keys_list, cpp],
     capture : true)
 
 fname = 'keyboard-keys-from-name.gperf'