Only use 'd' suffix when debug libraries are present. (#50)

Recent versions of Debian and Ubuntu (beginning with Stretch and Bionic
respectively) do not provide separate debug library versions of Poco.

The refactored debug check now actually verifies that a d-suffixed
library exists. If not it falls back to using the non-suffixed version
of the library which may or may not include debug symbols.

cmake/Modules/FindPoco.cmake

@@ -159,23 +159,26 @@ IF(Poco_INCLUDE_DIR)
         SET(Poco_BINARY_DIRS ${Poco_BINARY_DIR}/bin)
       ENDIF(Poco_BINARY_DIR AND EXISTS "${Poco_BINARY_DIR}/bin")
     ENDIF(Poco_LIBRARY_DIR AND Poco_FOUNDATION_LIB)
+    SET(DBG "")
     IF(Poco_FOUNDATION_LIB)
       IF ("${CMAKE_BUILD_TYPE}" STREQUAL "Debug")
-	SET(DBG "d")
-      ELSE ("${CMAKE_BUILD_TYPE}" STREQUAL "Debug")
-	SET(DBG "")
+          # Check that separate debug libraries are actually present.
+          FIND_LIBRARY(LIBFoundation "PocoFoundationd" Poco_LIBRARY_DIR)
+          IF(LIBFoundation)
+              SET(DBG "d")
+          ENDIF(LIBFoundation)
       ENDIF ("${CMAKE_BUILD_TYPE}" STREQUAL "Debug")
       SET(Comp_List "Foundation${DBG}")
       FOREACH(COMPONENT ${Poco_FIND_COMPONENTS})
-	FIND_LIBRARY(LIB${COMPONENT} "Poco${COMPONENT}${DBG}" Poco_LIBRARY_DIR)
-	IF (LIB${COMPONENT})
-	  LIST(APPEND Poco_LIBRARIES "${LIB${COMPONENT}}")
-	  LIST(APPEND Comp_List "${COMPONENT}${DBG}")
-	ENDIF(LIB${COMPONENT})
+          FIND_LIBRARY(LIB${COMPONENT} "Poco${COMPONENT}${DBG}" Poco_LIBRARY_DIR)
+          IF (LIB${COMPONENT})
+              LIST(APPEND Poco_LIBRARIES "${LIB${COMPONENT}}")
+              LIST(APPEND Comp_List "${COMPONENT}${DBG}")
+          ENDIF(LIB${COMPONENT})
       ENDFOREACH(COMPONENT)
       LIST(REMOVE_DUPLICATES Comp_List)
     ENDIF(Poco_FOUNDATION_LIB)
-    ENDIF(NOT Poco_LIBRARY_DIR)
+  ENDIF(NOT Poco_LIBRARY_DIR)
 ENDIF(Poco_INCLUDE_DIR)
 
 IF(NOT Poco_FOUND)