Fix dynamic librariy support detection for platforms without libdl

Some platforms (e.g. FreeBSD) do not have libdl, but dlopen and related
functions are part of libc. So first check for dlopen in libc, and only
if that fails, look for it in libdl.

Task-number: QTBUG-52951
Change-Id: I65a8ed18fce157da32f4e1ffeba30d7513385a8f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

config.tests/unix/dlopen/dlopen.pro

@@ -0,0 +1,2 @@
+SOURCES = $$PWD/dlopen.cpp
+CONFIG -= qt dylib

config.tests/unix/libdl/libdl.pro

@@ -1,3 +1,2 @@
-SOURCES = libdl.cpp
-CONFIG -= qt dylib
-!qnx: LIBS += -ldl
+include(../dlopen/dlopen.pro)
+LIBS += -ldl

configure

@@ -4743,9 +4743,13 @@ if [ "$CFG_LIBPNG" = "auto" ]; then
 fi
 
 # detect dl
-if ! compileTest unix/libdl "libdl"; then
-    QMakeVar add DEFINES QT_NO_DYNAMIC_LIBRARY
+if compileTest unix/dlopen "dlopen"; then
     QMAKE_CONFIG="$QMAKE_CONFIG no-libdl"
+else
+    if ! compileTest unix/libdl "libdl"; then
+        QMAKE_CONFIG="$QMAKE_CONFIG no-libdl"
+        QMakeVar add DEFINES QT_NO_DYNAMIC_LIBRARY
+    fi
 fi
 
 if [ "$CFG_EGLFS" = "yes" ]; then