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; }'

src/basic/generate-arphrd-list.sh

@@ -1,5 +1,5 @@
 #!/bin/sh -e
 
-cpp -dM -include net/if_arp.h - </dev/null | \
+$1 -dM -include net/if_arp.h - </dev/null | \
     awk '/^#define[ \t]+ARPHRD_[^ \t]+[ \t]+[^ \t]/ { print $2; }' | \
     sed -e 's/ARPHRD_//'

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

src/basic/generate-errno-list.sh

@@ -1,4 +1,4 @@
 #!/bin/sh -e
 
-cpp -dM -include errno.h - </dev/null | \
+$1 -dM -include errno.h - </dev/null | \
     awk '/^#define[ \t]+E[^ _]+[ \t]+/ { print $2; }'

src/basic/meson.build

@@ -206,28 +206,28 @@ generate_af_list = find_program('generate-af-list.sh')
 af_list_txt = custom_target(
     'af-list.txt',
     output : 'af-list.txt',
-    command : [generate_af_list],
+    command : [generate_af_list, cpp],
     capture : true)
 
 generate_arphrd_list = find_program('generate-arphrd-list.sh')
 arphrd_list_txt = custom_target(
     'arphrd-list.txt',
     output : 'arphrd-list.txt',
-    command : [generate_arphrd_list],
+    command : [generate_arphrd_list, cpp],
     capture : true)
 
 generate_cap_list = find_program('generate-cap-list.sh')
 cap_list_txt = custom_target(
     'cap-list.txt',
     output : 'cap-list.txt',
-    command : [generate_cap_list, config_h, missing_h],
+    command : [generate_cap_list, cpp, config_h, missing_h],
     capture : true)
 
 generate_errno_list = find_program('generate-errno-list.sh')
 errno_list_txt = custom_target(
     'errno-list.txt',
     output : 'errno-list.txt',
-    command : [generate_errno_list],
+    command : [generate_errno_list, cpp],
     capture : true)
 
 generated_gperf_headers = []