Fix unoptimized libraries in depends

This was a bug we inherited from BTC which causes some dependencies to
be built without optimization essentially replacing -O2 -pipe with
-O0 because cxxflags is assigned to instead of appended to.

We no longer have zeromq so for us this fixes the fact that bdb is being
compiled without optimization.

https://github.com/bitcoin/bitcoin/pull/22840

depends/packages/bdb.mk

@@ -14,7 +14,7 @@ ifneq ($(build_os),darwin)
 $(package)_config_opts_darwin=--disable-atomicsupport
 endif
 $(package)_config_opts_aarch64=--disable-atomicsupport
-$(package)_cxxflags=-std=c++11
+$(package)_cxxflags+=-std=c++11
 endef
 
 define $(package)_preprocess_cmds