Makefile: fix defines on ARM macOS

[cho-m]

Due to Darwin-arm64 conditional setting of CPPFLAGS, the subsequent CPPFLAGS += is resolved earlier on ARM macOS which results in empty paths being used.

Changelog-None

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Snippets of running make:

• Before commit:

  CC: cc -I/opt/homebrew/opt/sqlite/include -DCLN_NEXT_VERSION="v24.11" -DPKGLIBEXECDIR="" -DBINDIR="" -DPLUGINDIR="" -DCCAN_TAL_NEVER_RETURN_NULL=1 -Wall -Wundef -Wmissing-prototypes -Wmissing-declarations -Wstrict-prototypes -Wold-style-definition -Werror -std=gnu11 -g -fstack-protector-strong -Og -I ccan -I external/libwally-core/include/ -I external/libwally-core/src/secp256k1/include/ -I external/jsmn/ -I external/libbacktrace/ -I external/gheap/ -I external/build-arm64-apple-darwin24.1.0/libbacktrace-build -I . -I/opt/homebrew/include -I/opt/homebrew/opt/sqlite/include     -DSHACHAIN_BITS=48 -DJSMN_PARENT_LINKS  -DCOMPAT_V052=1 -DCOMPAT_V060=1 -DCOMPAT_V061=1 -DCOMPAT_V062=1 -DCOMPAT_V070=1 -DCOMPAT_V072=1 -DCOMPAT_V073=1 -DCOMPAT_V080=1 -DCOMPAT_V081=1 -DCOMPAT_V082=1 -DCOMPAT_V090=1 -DCOMPAT_V0100=1 -DCOMPAT_V0121=1  -c -o
  

• Just commenting out CPPFLAGS :=,

  CC: cc -DCLN_NEXT_VERSION="v24.11" -DPKGLIBEXECDIR="/usr/local/libexec/c-lightning" -DBINDIR="/usr/local/bin" -DPLUGINDIR="/usr/local/libexec/c-lightning/plugins" -DCCAN_TAL_NEVER_RETURN_NULL=1 -Wall -Wundef -Wmissing-prototypes -Wmissing-declarations -Wstrict-prototypes -Wold-style-definition -Werror -std=gnu11 -g -fstack-protector-strong -Og -I ccan -I external/libwally-core/include/ -I external/libwally-core/src/secp256k1/include/ -I external/jsmn/ -I external/libbacktrace/ -I external/gheap/ -I external/build-arm64-apple-darwin24.1.0/libbacktrace-build -I . -I/opt/homebrew/include -I/opt/homebrew/opt/sqlite/include     -DSHACHAIN_BITS=48 -DJSMN_PARENT_LINKS  -DCOMPAT_V052=1 -DCOMPAT_V060=1 -DCOMPAT_V061=1 -DCOMPAT_V062=1 -DCOMPAT_V070=1 -DCOMPAT_V072=1 -DCOMPAT_V073=1 -DCOMPAT_V080=1 -DCOMPAT_V081=1 -DCOMPAT_V082=1 -DCOMPAT_V090=1 -DCOMPAT_V0100=1 -DCOMPAT_V0121=1  -c -o
  

• After commit:

  CC: cc -I/opt/homebrew/opt/sqlite/include -DCLN_NEXT_VERSION="v24.11" -DPKGLIBEXECDIR="/usr/local/libexec/c-lightning" -DBINDIR="/usr/local/bin" -DPLUGINDIR="/usr/local/libexec/c-lightning/plugins" -DCCAN_TAL_NEVER_RETURN_NULL=1 -Wall -Wundef -Wmissing-prototypes -Wmissing-declarations -Wstrict-prototypes -Wold-style-definition -Werror -std=gnu11 -g -fstack-protector-strong -Og -I ccan -I external/libwally-core/include/ -I external/libwally-core/src/secp256k1/include/ -I external/jsmn/ -I external/libbacktrace/ -I external/gheap/ -I external/build-arm64-apple-darwin24.1.0/libbacktrace-build -I . -I/opt/homebrew/include -I/opt/homebrew/opt/sqlite/include     -DSHACHAIN_BITS=48 -DJSMN_PARENT_LINKS  -DCOMPAT_V052=1 -DCOMPAT_V060=1 -DCOMPAT_V061=1 -DCOMPAT_V062=1 -DCOMPAT_V070=1 -DCOMPAT_V072=1 -DCOMPAT_V073=1 -DCOMPAT_V080=1 -DCOMPAT_V081=1 -DCOMPAT_V082=1 -DCOMPAT_V090=1 -DCOMPAT_V0100=1 -DCOMPAT_V0121=1  -c -o
  

[rustyrussell]

Thanks!