tree-wide: several cleanups for generating symbol lists and gperf files

- pass our system include directories to make generators use our libc
  wrappers and latest kernel headers,
- include relevant headers in generated gperf file,
- use files() rather than find_program(), as the result of
  find_program() cannot be passed to 'input' of custom_target(),
- move generate-bpf-delegate-configs.py to src/core/, as it is only used
  by libcore.

src/include/meson.build

@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+# Source files that provides AF_XYZ
+af_sources = files(
+        'override/sys/socket.h',
+)
+
+# Source files that provides ARPHRD_XYZ
+arphrd_sources = files(
+        'uapi/linux/if_arp.h',
+)
+
+# Source files that provides CAP_XYZ
+cap_sources = files(
+        'uapi/linux/capability.h',
+)
+
+# Source files that provides BPF delegate options
+bpf_delegate_sources = files(
+        'uapi/linux/bpf.h',
+)
+
+# Source files that provides ETHTOOL_LINK_MODE_XYZ
+ethtool_link_mode_sources = files(
+        'uapi/linux/ethtool.h',
+)
+
+# Source files that provides IPPROTO_XYZ
+ipproto_sources = files(
+        'override/netinet/in.h',
+        'uapi/linux/in.h',
+)
+
+# Source files that provides KEY_XYZ
+keyboard_sources = files(
+        'uapi/linux/input.h',
+        'uapi/linux/input-event-codes.h',
+)