This extends the ExportImport test. The Export project creates a C++
static library and exports it. Then the Import project links the
library into a C executable. On most platforms the executable will link
only if the C++ linker is chosen correctly.
Linking to a Windows shared library (.dll) requires only its import
library (.lib). This teaches CMake to recognize SHARED IMPORTED library
targets that set only IMPORTED_IMPLIB and not IMPORTED_LOCATION.
Brad King