Properly support -DENABLE_REDIS=TRUE even if library is not found

.travis.yml

@@ -3,7 +3,15 @@ compiler:
   - gcc
   - clang
 dist: bionic
-before_install: sudo apt-get install -y cmake libgd-dev libsqlite3-dev libleveldb-dev
+addons:
+  apt:
+    packages:
+      - cmake
+      - libgd-dev
+      - libsqlite3-dev
+      - libleveldb-dev
+      - libpq-dev
+      - postgresql-server-dev-all
 script: ./util/travis/script.sh
 notifications:
   email: false

CMakeLists.txt

@@ -95,6 +95,7 @@ if(ENABLE_POSTGRESQL)
 		include_directories(${PostgreSQL_INCLUDE_DIRS})
 	else()
 		message(STATUS "PostgreSQL not found!")
+		set(PostgreSQL_LIBRARIES "")
 	endif()
 endif(ENABLE_POSTGRESQL)
 
@@ -112,9 +113,10 @@ if(ENABLE_LEVELDB)
 		set(USE_LEVELDB TRUE)
 		message(STATUS "LevelDB backend enabled")
 		include_directories(${LEVELDB_INCLUDE_DIR})
-	else(LEVELDB_LIBRARY AND LEVELDB_INCLUDE_DIR)
+	else()
 		message(STATUS "LevelDB not found!")
-	endif(LEVELDB_LIBRARY AND LEVELDB_INCLUDE_DIR)
+		set(LEVELDB_LIBRARY "")
+	endif()
 endif(ENABLE_LEVELDB)
 
 # Libraries: redis
@@ -131,9 +133,10 @@ if(ENABLE_REDIS)
 		set(USE_REDIS TRUE)
 		message(STATUS "redis backend enabled")
 		include_directories(${REDIS_INCLUDE_DIR})
-	else(REDIS_LIBRARY AND REDIS_INCLUDE_DIR)
+	else()
 		message(STATUS "redis not found!")
-	endif(REDIS_LIBRARY AND REDIS_INCLUDE_DIR)
+		set(REDIS_LIBRARY "")
+	endif()
 endif(ENABLE_REDIS)
 
 # Compiling & Linking