depends: Add CMAKE_SYSTEM_VERSION to CMake toolchain file

depends/Makefile

@@ -268,6 +268,7 @@ $(host_prefix)/toolchain.cmake : toolchain.cmake.in $(host_prefix)/.stamp_$(fina
 	@mkdir -p $(@D)
 	sed -e 's|@depends_crosscompiling@|$(crosscompiling)|' \
             -e 's|@host_system_name@|$($(host_os)_cmake_system_name)|' \
+            -e 's|@host_system_version@|$($(host_os)_cmake_system_version)|' \
             -e 's|@host_arch@|$(host_arch)|' \
             -e 's|@CC@|$(host_CC)|' \
             -e 's|@CXX@|$(host_CXX)|' \

depends/hosts/darwin.mk

@@ -109,3 +109,6 @@ darwin_debug_CFLAGS=-O1 -g
 darwin_debug_CXXFLAGS=$(darwin_debug_CFLAGS)
 
 darwin_cmake_system_name=Darwin
+# Darwin version, which corresponds to OSX_MIN_VERSION.
+# See https://en.wikipedia.org/wiki/Darwin_(operating_system)
+darwin_cmake_system_version=20.1

depends/hosts/linux.mk

@@ -39,4 +39,7 @@ i686_linux_CXX=$(default_host_CXX) -m32
 x86_64_linux_CC=$(default_host_CC) -m64
 x86_64_linux_CXX=$(default_host_CXX) -m64
 endif
+
 linux_cmake_system_name=Linux
+# Refer to doc/dependencies.md for the minimum required kernel.
+linux_cmake_system_version=3.17.0

depends/hosts/mingw32.mk

@@ -20,3 +20,5 @@ mingw32_debug_CXXFLAGS=$(mingw32_debug_CFLAGS)
 mingw32_debug_CPPFLAGS=-D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC
 
 mingw32_cmake_system_name=Windows
+# Windows 7 (NT 6.1).
+mingw32_cmake_system_version=6.1

depends/toolchain.cmake.in

@@ -6,6 +6,7 @@
 
 if(@depends_crosscompiling@)
   set(CMAKE_SYSTEM_NAME @host_system_name@)
+  set(CMAKE_SYSTEM_VERSION @host_system_version@)
   set(CMAKE_SYSTEM_PROCESSOR @host_arch@)
 endif()