build: set OSX_MIN_VERSION to 10.15

This is required to use std::filesystem on macOS as support for it only
landed in the libc++ dylib shipped with 10.15.

See also: https://developer.apple.com/documentation/xcode-release-notes/xcode-11-release-notes

Clang now supports the C++17 <filesystem> library for iOS 13, macOS 10.15, watchOS 6, and tvOS 13.

depends/hosts/darwin.mk

@@ -1,4 +1,4 @@
-OSX_MIN_VERSION=10.14
+OSX_MIN_VERSION=10.15
 OSX_SDK_VERSION=10.15.6
 XCODE_VERSION=12.1
 XCODE_BUILD_ID=12A7403