[welcome] Make libparted optional

- This turns off the space-available check in the welcome module;
   without libparted, always fail that check.
 - Allows running the welcome module on OS without libparted.

src/modules/welcome/CMakeLists.txt

@@ -1,20 +1,24 @@
 include_directories( ${PROJECT_BINARY_DIR}/src/libcalamaresui )
 
-find_package( LIBPARTED REQUIRED )
 find_package( Qt5 ${QT_VERSION} CONFIG REQUIRED DBus Network )
 
+find_package( LIBPARTED )
+if ( LIBPARTED_FOUND )
+    set( PARTMAN_SRC checker/partman_devices.c )
+    set( CHECKER_LINK_LIBRARIES ${LIBPARTED_LIBRARY} )
+else()
+    set( PARTMAN_SRC )
+    set( CHECKER_LINK_LIBRARIES )
+    add_definitions( -DWITHOUT_LIBPARTED )
+endif()
+
 include_directories( ${PROJECT_BINARY_DIR}/src/libcalamaresui )
 
 set( CHECKER_SOURCES
     checker/CheckItemWidget.cpp
     checker/CheckerWidget.cpp
     checker/RequirementsChecker.cpp
-    checker/partman_devices.c
-)
-set( CHECKER_LINK_LIBRARIES
-    ${LIBPARTED_LIBRARY}
-    Qt5::DBus
-    Qt5::Network
+    ${PARTMAN_SRC}
 )
 
 calamares_add_plugin( welcome
@@ -29,5 +33,7 @@ calamares_add_plugin( welcome
     LINK_PRIVATE_LIBRARIES
         calamaresui
         ${CHECKER_LINK_LIBRARIES}
+        Qt5::DBus
+        Qt5::Network
     SHARED_LIB
 )

src/modules/welcome/checker/RequirementsChecker.cpp

@@ -313,7 +313,12 @@ RequirementsChecker::verdict() const
 bool
 RequirementsChecker::checkEnoughStorage( qint64 requiredSpace )
 {
+#ifdef WITHOUT_LIBPARTED
+    cDebug() << "WARNING: RequirementsChecker is configured without libparted.";
+    return false;
+#else
     return check_big_enough( requiredSpace );
+#endif
 }