[issue1122] Specify using shared CPLEX libraries in Windows.

The previous fix to find CPLEX libraries in the absence of dynamic ones caused
Windows to not find CPLEX anymore. We fix this by distinguishing the type of
imported library depending on the operating system.

src/search/cmake/FindCplex.cmake

@@ -12,7 +12,16 @@
 set(IMPORTED_CONFIGURATIONS "Debug" "Release")
 set(HINT_PATHS ${cplex_DIR} $ENV{cplex_DIR})
 
-add_library(cplex::cplex IMPORTED UNKNOWN)
+if(WIN32)
+    # On Windows we have to declare the library as SHARED to correctly
+    # communicate the location of the dll and impllib files.
+    add_library(cplex::cplex IMPORTED SHARED)
+else()
+    # On Linux, the CPLEX installer sometimes does not provide dynamic
+    # libraries. If they are absent, we fall back to static ones further down,
+    # hence we mark the type unknown here.
+    add_library(cplex::cplex IMPORTED UNKNOWN)
+endif()
 set_target_properties(cplex::cplex PROPERTIES
     IMPORTED_CONFIGURATIONS "${IMPORTED_CONFIGURATIONS}"
 )