BUG: Avoid using def file when BUILD_SHARED_LIBS is off

Using the def file causes build errors like this with the Visual
Studio 11 linker when building static libraries:

  [2445/5676] Linking C static library lib\itktiff-4.4.lib
  FAILED: cmd.exe /c cd . && C:\PROGRA~2\MICROS~1.0\VC\bin\link.exe /lib /nologo /DEF:"C:\dev\...
  itktiff.def : error LNK2001: unresolved external symbol TIFFCheckTile

Since the def file is only for specifying exports in the shared
library build, it should be ok to leave it out for static library
builds.

Change-Id: Ic67f8b763b16b469a60064a6262653f2e6df13df
Merge topic 'avoid-def-file-for-static-builds'

bcd155e BUG: Avoid using def file when BUILD_SHARED_LIBS is off