build: add and use CXX_STANDARD in depends

depends/hosts/android.mk

@@ -7,6 +7,7 @@ android_CC=$(ANDROID_TOOLCHAIN_BIN)/$(HOST)$(ANDROID_API_LEVEL)-clang
 endif
 
 android_CFLAGS=-std=$(C_STANDARD)
+android_CXXFLAGS=-std=$(CXX_STANDARD)
 
 ifneq ($(LTO),)
 android_CFLAGS += -flto

depends/hosts/darwin.mk

@@ -110,7 +110,7 @@ darwin_CXX=env -u C_INCLUDE_PATH -u CPLUS_INCLUDE_PATH \
                -Xclang -internal-externc-isystem$(OSX_SDK)/usr/include
 
 darwin_CFLAGS=-pipe -std=$(C_STANDARD)
-darwin_CXXFLAGS=-pipe
+darwin_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
 ifneq ($(LTO),)
 darwin_CFLAGS += -flto

depends/hosts/freebsd.mk

@@ -1,5 +1,5 @@
 freebsd_CFLAGS=-pipe -std=$(C_STANDARD)
-freebsd_CXXFLAGS=-pipe
+freebsd_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
 ifneq ($(LTO),)
 freebsd_CFLAGS += -flto

depends/hosts/linux.mk

@@ -1,5 +1,5 @@
 linux_CFLAGS=-pipe -std=$(C_STANDARD)
-linux_CXXFLAGS=-pipe
+linux_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
 ifneq ($(LTO),)
 linux_CFLAGS += -flto

depends/hosts/mingw32.mk

@@ -3,7 +3,7 @@ mingw32_CXX := $(host)-g++-posix
 endif
 
 mingw32_CFLAGS=-pipe -std=$(C_STANDARD)
-mingw32_CXXFLAGS=-pipe
+mingw32_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
 ifneq ($(LTO),)
 mingw32_CFLAGS += -flto

depends/hosts/netbsd.mk

@@ -1,5 +1,5 @@
 netbsd_CFLAGS=-pipe -std=$(C_STANDARD)
-netbsd_CXXFLAGS=-pipe
+netbsd_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
 ifneq ($(LTO),)
 netbsd_CFLAGS += -flto

depends/hosts/openbsd.mk

@@ -1,5 +1,5 @@
 openbsd_CFLAGS=-pipe -std=$(C_STANDARD)
-openbsd_CXXFLAGS=-pipe
+openbsd_CXXFLAGS=-pipe -std=$(CXX_STANDARD)
 
 ifneq ($(LTO),)
 openbsd_CFLAGS += -flto