Merge bitcoin/bitcoin#23619: build: Propagate user-defined flags to host packages

a3a2bd9 ci: Drop no longer needed package-specific flags (Hennadii Stepanov)
071eef1 build: Propagate user-defined flags to host packages (Hennadii Stepanov)

Pull request description:

  On master (4f8b1f8) `{CPP,C,CXX,LD}FLAGS` that are specified in the command line are not propagated to packages:
  ```
  $ make --no-print-directory -C depends print-libevent_cxxflags CXXFLAGS=-some-fancy-flag
  libevent_cxxflags=-pipe -O2
  ```

  This PR:
  - propagates `{CPP,C,CXX,LD}FLAGS` to host packages:
  ```
  $ make --no-print-directory -C depends print-libevent_cxxflags CXXFLAGS=-some-fancy-flag
  libevent_cxxflags= -some-fancy-flag
  ```
  - does not propagate `{CPP,C,CXX,LD}FLAGS` to native packages:
  ```
  $ make --no-print-directory -C depends print-native_b2_cxxflags CXXFLAGS=-some-fancy-flag
  native_b2_cxxflags=
  ```
  - actually addresses the bitcoin/bitcoin#23551 (comment)

ACKs for top commit:
  TheCharlatan:
    Code review ACK a3a2bd9

Tree-SHA512: 243d6b1b0e9c5de46debc36de62a77b6b4d6f638940fd530040c219956ec624e321b0c25290fed164e3a8c88befa7b97b20f765d7b9a428c269b3720f21da099

Author: fanquake

ci/test/00_setup_env_native_fuzz_with_msan.sh

@@ -15,7 +15,7 @@ export MSAN_AND_LIBCXX_FLAGS="${MSAN_FLAGS} ${LIBCXX_FLAGS}"
 export CONTAINER_NAME="ci_native_fuzz_msan"
 export PACKAGES="clang-12 llvm-12 cmake"
 # BDB generates false-positives and will be removed in future
-export DEP_OPTS="NO_BDB=1 NO_QT=1 CC='clang' CXX='clang++' CFLAGS='${MSAN_FLAGS}' CXXFLAGS='${MSAN_AND_LIBCXX_FLAGS}' libevent_cflags='${MSAN_FLAGS}' sqlite_cflags='${MSAN_FLAGS}' zeromq_cxxflags='-std=c++17 ${MSAN_AND_LIBCXX_FLAGS}'"
+export DEP_OPTS="NO_BDB=1 NO_QT=1 CC='clang' CXX='clang++' CFLAGS='${MSAN_FLAGS}' CXXFLAGS='${MSAN_AND_LIBCXX_FLAGS}'"
 export GOAL="install"
 export BITCOIN_CONFIG="--enable-fuzz --with-sanitizers=fuzzer,memory --disable-hardening --with-asm=no CC=clang CXX=clang++ CFLAGS='${MSAN_FLAGS}' CXXFLAGS='${MSAN_AND_LIBCXX_FLAGS}'"
 export USE_MEMORY_SANITIZER="true"

ci/test/00_setup_env_native_msan.sh

@@ -15,7 +15,7 @@ export MSAN_AND_LIBCXX_FLAGS="${MSAN_FLAGS} ${LIBCXX_FLAGS}"
 export CONTAINER_NAME="ci_native_msan"
 export PACKAGES="clang-12 llvm-12 cmake"
 # BDB generates false-positives and will be removed in future
-export DEP_OPTS="NO_BDB=1 NO_QT=1 CC='clang' CXX='clang++' CFLAGS='${MSAN_FLAGS}' CXXFLAGS='${MSAN_AND_LIBCXX_FLAGS}' libevent_cflags='${MSAN_FLAGS}' sqlite_cflags='${MSAN_FLAGS}' zeromq_cxxflags='-std=c++17 ${MSAN_AND_LIBCXX_FLAGS}'"
+export DEP_OPTS="NO_BDB=1 NO_QT=1 CC='clang' CXX='clang++' CFLAGS='${MSAN_FLAGS}' CXXFLAGS='${MSAN_AND_LIBCXX_FLAGS}'"
 export GOAL="install"
 export BITCOIN_CONFIG="--with-sanitizers=memory --disable-hardening --with-asm=no CC=clang CXX=clang++ CFLAGS='${MSAN_FLAGS}' CXXFLAGS='${MSAN_AND_LIBCXX_FLAGS}'"
 export USE_MEMORY_SANITIZER="true"

depends/hosts/default.mk

@@ -28,8 +28,13 @@ host_$1=$$($(host_arch)_$(host_os)_$1)
 endef
 
 define add_host_flags_func
+ifeq ($(filter $(origin $1),undefined default),)
+$(host_arch)_$(host_os)_$1 =
+$(host_arch)_$(host_os)_$(release_type)_$1 = $($1)
+else
 $(host_arch)_$(host_os)_$1 += $($(host_os)_$1)
 $(host_arch)_$(host_os)_$(release_type)_$1 += $($(host_os)_$(release_type)_$1)
+endif
 host_$1 = $$($(host_arch)_$(host_os)_$1)
 host_$(release_type)_$1 = $$($(host_arch)_$(host_os)_$(release_type)_$1)
 endef