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/bdb.mk

@@ -12,7 +12,7 @@ $(package)_config_opts_mingw32=--enable-mingw
 $(package)_config_opts_linux=--with-pic
 $(package)_config_opts_android=--with-pic
 $(package)_cflags+=-Wno-error=implicit-function-declaration
-$(package)_cxxflags=-std=c++17
+$(package)_cxxflags+=-std=c++17
 $(package)_cppflags_mingw32=-DUNICODE -D_UNICODE
 endef
 

depends/packages/boost.mk

@@ -23,7 +23,7 @@ else
 $(package)_toolset_$(host_os)=gcc
 endif
 $(package)_config_libraries=filesystem,system,test
-$(package)_cxxflags=-std=c++17 -fvisibility=hidden
+$(package)_cxxflags+=-std=c++17 -fvisibility=hidden
 $(package)_cxxflags_linux=-fPIC
 $(package)_cxxflags_android=-fPIC
 $(package)_cxxflags_x86_64_darwin=-fcf-protection=full

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