depends: Add host-specific `cmake_system_version` variables

depends/hosts/darwin.mk

@@ -76,3 +76,6 @@ 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