build: fix unoptimized libraries in depends

We need to append-to rather than set CXXFLAGS, otherwise we loose -O2 &
-pipe. Currently this results in zeromq being built without optimizations
at all (or whatever the compiler would default too, essentially always -O0).

Bdb is the same, for the CXX portion of its code. C code has been built
with -O2.

Boost has actually been uneffected because it receives -O3 from it's own
build flags.

depends/packages/zeromq.mk

@@ -12,7 +12,7 @@ define $(package)_set_vars
   $(package)_config_opts += --disable-Werror --disable-drafts --enable-option-checking
   $(package)_config_opts_linux=--with-pic
   $(package)_config_opts_android=--with-pic
-  $(package)_cxxflags=-std=c++17
+  $(package)_cxxflags+=-std=c++17
 endef
 
 define $(package)_preprocess_cmds