cmake: do not require static deps for a shared library

Commit a8c3455,
commit 5e1cf4d, and
commit 4dfb781 implement & add the
libraries needed in case of static linking as dependencies for the cmake
config file. There are two problems with the implementation:
- being unconditionally there, they will make those modules required
  when building against exiv2, even when not building statically
- the exiv2 library build is done either shared or static, so there is
  no static library in case of a shared build

Hence, require the dependencies for the static linking in the cmake
config file only when the exiv2 library build is not shared (i.e.
static).

cmake/exiv2Config.cmake.in

@@ -3,12 +3,14 @@
 cmake_minimum_required(VERSION 3.5)
 include(CMakeFindDependencyMacro)
 
-if(@EXIV2_ENABLE_PNG@) # if(EXIV2_ENABLE_PNG)
-  find_dependency(ZLIB REQUIRED)
-endif()
+if(NOT @BUILD_SHARED_LIBS@) # if(NOT BUILD_SHARED_LIBS)
+  if(@EXIV2_ENABLE_PNG@) # if(EXIV2_ENABLE_PNG)
+    find_dependency(ZLIB REQUIRED)
+  endif()
 
-if(@EXIV2_ENABLE_XMP@) # if(EXIV2_ENABLE_XMP)
-  find_dependency(EXPAT REQUIRED)
+  if(@EXIV2_ENABLE_XMP@) # if(EXIV2_ENABLE_XMP)
+    find_dependency(EXPAT REQUIRED)
+  endif()
 endif()
 
 include("${CMAKE_CURRENT_LIST_DIR}/exiv2Export.cmake")