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.

contrib/guix/guix-build

@@ -76,7 +76,7 @@ mkdir -p "$VERSION_BASE"
 # Default to building for all supported HOSTs (overridable by environment)
 export HOSTS="${HOSTS:-x86_64-linux-gnu arm-linux-gnueabihf aarch64-linux-gnu riscv64-linux-gnu powerpc64-linux-gnu powerpc64le-linux-gnu
                        x86_64-w64-mingw32
-                       x86_64-apple-darwin18}"
+                       x86_64-apple-darwin19}"
 
 # Usage: distsrc_for_host HOST
 #