Merge bitcoin/bitcoin#23948: build: add support for FreeBSD to depends

ae9175f608 build: add FreeBSD support to depends (fanquake)

Pull request description:

  Setup to use the system Clang (11.0.1 as of FreeBSD 13.0).

  Doesn't build the Qt package; that requires a few additional changes. The current issue is that FreeBSDs `byacc` seems to have an issue parsing something in libxkbcommon. Work in progress branch here: https://github.com/fanquake/bitcoin/tree/depends_support_freebsd_qt. In any case, I don't think building the Qt libs on FreeBSD is a super high priority (I'd also have no way to test the GUI).

ACKs for top commit:
  laanwj:
    ACK ae9175f608

Tree-SHA512: 1ecc7855f0195f68c96e534bd77ce17c3975db1dfb3caa35302c2b46643c8c30f75b1c2e1735cf69f1eddb70447d11e67e7f339ef5497336cdff7a59b32be961

depends/packages/boost.mk

@@ -25,6 +25,7 @@ endif
 $(package)_config_libraries=filesystem,system,test
 $(package)_cxxflags+=-std=c++17
 $(package)_cxxflags_linux=-fPIC
+$(package)_cxxflags_freebsd=-fPIC
 $(package)_cxxflags_android=-fPIC
 $(package)_cxxflags_x86_64=-fcf-protection=full
 endef