configure: add support for multiple libdirs in inline library sources

while the command line doesn't actually permit it (that can be
re-evaluated separately), derivatives of the inline source type may want
to inject additional paths, as is the case with opcua.

the incdir field supports multiple entries without additional action.

Change-Id: I3860ca1fc8fab25c04eb63bdb2f855b77ff3b9a4
Reviewed-by: Rainer Keller <Rainer.Keller@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

mkspecs/features/qt_configure.prf

@@ -707,8 +707,12 @@ defineTest(qtConfLibrary_inline) {
     }
 
     libdir = $$eval(config.input.$${input}.libdir)
-    !isEmpty(libdir): \
-        $${1}.libs = -L$$libdir $$eval($${1}.libs)
+    !isEmpty(libdir) {
+        libs =
+        for (ld, libdir): \
+            libs += -L$$ld
+        $${1}.libs = $$libs $$eval($${1}.libs)
+    }
 
     !qtConfResolveAllLibs($$1): \
         return(false)