qmake: Don't error out if QMAKE_DEFAULT_LIBDIRS is empty on macOS

The new linker in Xcode 15 doesn't provide any default linker or
framework paths when requested via -v, but still seems to use the
default paths documented in the ld man page.

We trust that linker will do the right thing, even if we don't
know of its default linker paths.

We also need to opt out of the default fallback logic to
set the libdirs to /lib and /usr/lib.

This may result in UnixMakefileGenerator::findLibraries finding
different libraries than expected, if additional paths are
passed with -L, which will then take precedence for qmake,
even if the linker itself will use the library from the
SDK's default paths. This should hopefully not be an issue
in practice, as we don't turn -lFoo into absolute paths in
qmake, so the only risk is that we're picking up the wrong
prl files and adding additional dependencies that the lib
in the SDK doesn't have.

Upstream commits:
 - Qt 5.15.16: Not yet publicly available.
 - Qt dev: cdf64b0e47115cc473e1afd1472b4b09e130b2a5

For other Qt branches see
https://codereview.qt-project.org/q/I2347b26e2df0828471373b0e15b8c9089274c65d

--- old/qtbase/mkspecs/features/toolchain.prf
+++ new/qtbase/mkspecs/features/toolchain.prf
@@ -283,9 +283,12 @@ isEmpty($${target_prefix}.INCDIRS) {
                 }
             }
         }
-        isEmpty(QMAKE_DEFAULT_LIBDIRS)|isEmpty(QMAKE_DEFAULT_INCDIRS): \
+        isEmpty(QMAKE_DEFAULT_INCDIRS): \
             !integrity: \
-                error("failed to parse default search paths from compiler output")
+                error("failed to parse default include paths from compiler output")
+        isEmpty(QMAKE_DEFAULT_LIBDIRS): \
+            !integrity:!darwin: \
+                error("failed to parse default library paths from compiler output")
         QMAKE_DEFAULT_LIBDIRS = $$unique(QMAKE_DEFAULT_LIBDIRS)
     } else: ghs {
         cmd = $$QMAKE_CXX $$QMAKE_CXXFLAGS -$${LITERAL_HASH} -o /tmp/fake_output /tmp/fake_input.cpp
@@ -407,7 +410,7 @@ isEmpty($${target_prefix}.INCDIRS) {
         QMAKE_DEFAULT_INCDIRS = $$split(INCLUDE, $$QMAKE_DIRLIST_SEP)
     }
 
-    unix:if(!cross_compile|host_build) {
+    unix:!darwin:if(!cross_compile|host_build) {
         isEmpty(QMAKE_DEFAULT_INCDIRS): QMAKE_DEFAULT_INCDIRS = /usr/include /usr/local/include
         isEmpty(QMAKE_DEFAULT_LIBDIRS): QMAKE_DEFAULT_LIBDIRS = /lib /usr/lib
     }