Utility: make Debug::isTty() more reliable on macOS.

doc/corrade-changelog.dox

@@ -254,6 +254,15 @@ namespace Corrade {
     if one of them failed to open (see [mosra/corrade#70](https://github.com/mosra/corrade/pull/70))
 -   @ref Interconnect::Emitter::emit() (and signal/slot connections in general)
     now work correctly with non-copyable arguments
+-   Fixed @ref Utility::Debug::isTty() (and thus also @ref TestSuite colored
+    output) to reliably work on macOS. Because Xcode output window doesn't
+    support ANSI color escape codes, tt was relying on an undocumented
+    @cb{.sh} $XPC_SERVICE_NAME @ce variable that was always defined inside
+    Xcode to disable colored output. Nowadays this variable is sometimes
+    defined outside as well, making the check unreliable. Fixed by testing for
+    @cb{.sh} $TERM @ce instead, see
+    [mosra/corrade#73](https://github.com/mosra/corrade/issues/73) for more
+    information.
 
 @subsection corrade-changelog-latest-deprecated Deprecated APIs
 

src/Corrade/Utility/Debug.cpp

@@ -237,9 +237,12 @@ bool Debug::isTty(std::ostream* const output) {
         #endif
         #ifdef CORRADE_TARGET_APPLE
         /* Xcode's console reports that it is a TTY, but it doesn't support
-           colors. We have to check for the following undocumented environment
-           variable instead. If set, then don't use colors. */
-        && !std::getenv("XPC_SERVICE_NAME")
+           colors. Originally this was testing for XPC_SERVICE_NAME being
+           defined because that's always defined when running inside Xcode, but
+           nowadays that's often defined also outside of Xcode, so it's
+           useless. According to https://stackoverflow.com/a/39292112, we can
+           reliably check for TERM -- if it's null, we're inside Xcode. */
+        && std::getenv("TERM")
         #endif
         ;