qmake: fix QMAKE_DEFAULT_LIBDIRS detection on android

amazingly enough, android has different sysroots for the compiler
(shared includes full of #ifdefs) and the linker (per-platform
libraries).

this patch supports only clang for non-darwin, which notably covers all
supported android ndks.

with this fixed, we also remove the hard-coded setting of
QMAKE_DEFAULT_*DIRS from the specs.

amends 353fb118c.

Change-Id: Ie0513de0f7123d7f5b8ca1ffcc72c017cddd126c
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

mkspecs/common/android-base-tail.conf

@@ -87,6 +87,3 @@ QMAKE_LIBS_OPENGL_ES2   = -lGLESv2
 !exists($$NDK_ROOT): error("You need to set the ANDROID_NDK_ROOT environment variable to point to your Android NDK.")
 
 load(qt_config)
-
-QMAKE_DEFAULT_LIBDIRS = $$QMAKE_LIBDIR
-QMAKE_DEFAULT_INCDIRS = $$QMAKE_INCDIR

mkspecs/features/toolchain.prf

@@ -128,8 +128,8 @@ isEmpty($${target_prefix}.INCDIRS) {
             # paths, so it can't just be used in place of the above code).
             # What's more, -print-search-dirs can't be used on clang on Apple because it
             # won't print all the library paths (only the clang-internal ones).
-            output = $$system("$$cmd_prefix $$QMAKE_CXX -print-search-dirs", lines, ec)
-            !equals(ec, 0): qtCompilerErrror($$QMAKE_CXX, $$output)
+            output = $$system("$$cmd_prefix $$QMAKE_LINK $$QMAKE_LFLAGS -print-search-dirs", lines, ec)
+            !equals(ec, 0): qtCompilerErrror($$QMAKE_LINK, $$output)
 
             for (line, output) {
                 contains(line, "^libraries: .*") {