Enable web view support only by default if Qt WebEngine is present

The web view is only an optional enhancement in my applications so having
the support disabled is rather common. So for a better out-of-the-box
experience when configuring my projects it makes sense not having to
disable web view support explicitly if Qt WebEngine is not installed
anyways.

cmake/modules/QtConfig.cmake

@@ -66,10 +66,6 @@ if (IMPORTED_KF_MODULES)
 endif ()
 
 # find and use the required Qt/KF modules
-set(QT_PACKAGE_PREFIX
-    "Qt5"
-    CACHE STRING "specifies the prefix for Qt packages")
-string(TOLOWER "${QT_PACKAGE_PREFIX}" QT_PACKAGE_PREFIX_LOWER)
 set(QT_LINGUIST_TOOLS_PACKAGE "${QT_PACKAGE_PREFIX}LinguistTools")
 set(QT_QMAKE_TARGET "${QT_PACKAGE_PREFIX}::qmake")
 foreach (MODULE ${QT_MODULES})
@@ -79,9 +75,6 @@ foreach (MODULE ${QT_MODULES})
     endif ()
     use_qt_module(PREFIX "${QT_PACKAGE_PREFIX}" MODULE "${MODULE}" ${MODULE_OPTIONS})
 endforeach ()
-set(KF_PACKAGE_PREFIX
-    "KF5"
-    CACHE STRING "specifies the prefix for KDE Frameworks packages")
 foreach (MODULE ${KF_MODULES})
     unset(MODULE_OPTIONS)
     if ("${MODULE}" IN_LIST META_PUBLIC_KF_MODULES)
@@ -329,6 +322,7 @@ option(BUILTIN_TRANSLATIONS_OF_QT "enables/disables built-in translations of Qt
        "${BUILTIN_TRANSLATIONS}")
 
 # determine relevant Qt translation files
+string(TOLOWER "${QT_PACKAGE_PREFIX}" QT_PACKAGE_PREFIX_LOWER)
 set(QT_TRANSLATION_FILES)
 set(QT_TRANSLATION_SEARCH_PATHS)
 query_qmake_variable_path(QT_INSTALL_TRANSLATIONS)

cmake/modules/QtLinkage.cmake

@@ -11,6 +11,14 @@ set(QT_LINKAGE_DETERMINED ON)
 # include validate_visibility from c++utilities' 3rdParty module
 include(3rdParty)
 
+# allow switching the Qt and KDE Frameworks version
+set(QT_PACKAGE_PREFIX
+    "Qt5"
+    CACHE STRING "specifies the prefix for Qt packages")
+set(KF_PACKAGE_PREFIX
+    "KF5"
+    CACHE STRING "specifies the prefix for KDE Frameworks packages")
+
 # determine the minimum Qt version
 if (NOT META_QT_VERSION)
     if (META_QT5_VERSION)

cmake/modules/QtWebViewProviderConfig.cmake

@@ -6,9 +6,19 @@ if (TARGET_CONFIG_DONE)
     message(FATAL_ERROR "Can not include QtWebViewProviderConfig module when targets are already configured.")
 endif ()
 
+# include required modules
+include(QtLinkage)
+
+# check whether Qt WebEngine is present
+find_package("${QT_PACKAGE_PREFIX}WebEngineWidgets" "${META_QT_VERSION}")
+set(WEBVIEW_PROVIDER_DEFAULT "none")
+if ("${${QT_PACKAGE_PREFIX}WebEngineWidgets_FOUND}")
+    set(WEBVIEW_PROVIDER_DEFAULT "webengine")
+endif ()
+
 # configure the specified web view provider
 set(WEBVIEW_PROVIDER
-    "webengine"
+    "${WEBVIEW_PROVIDER_DEFAULT}"
     CACHE STRING "specifies the web view provider: webengine (default), webkit or none")
 if (WEBVIEW_PROVIDER STREQUAL "webkit")
     set(WEBVIEW_PROVIDER WebKitWidgets)