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.

share/qt/Info.plist.in

@@ -3,7 +3,7 @@
 <plist version="0.9">
 <dict>
   <key>LSMinimumSystemVersion</key>
-  <string>10.14.0</string>
+  <string>10.15.0</string>
 
   <key>LSArchitecturePriority</key>
   <array>