Link with -ldl option only when it is supported

-ldl option was used unconditionally while libdl is not supported
when libc is static.
Add build test to configure which checks if libdl is supported.
QMAKE_LIBS_DYNLOAD in "src/corelib/plugin/plugin.pri" is now used only
if libdl is available.
qt_linux_find_symbol_sys from qlibrary_unix is now used only if
QT_NO_DYNAMIC_LIBRARY is not defined.

Initially reported by Buildroot autobuilder here:
http://autobuild.buildroot.net/results/a85/a85a1839a45fb6102e53131ecc8f6dadf92bcdc2



Change-Id: I0397472456efdc4f3ab5f24d01253bee8048a9d1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>