Log Message:
gwenview: fix build with newer exiv2. workaround cryptic cmake issue.

Bypass FindExiv2.cmake with pkgconfig. Use the big exiv2 header instead
of two headers that are no longer sufficient.

For some reason the test CMAKE_COMPILER_IS_GNUCXX doesn't pass, so
we don't know how to enable/disable exceptions. Use BUILDLINK_TRANSFORM
for this.