Do not download sources during a Gentoo package build.

Rather than trying to access the network, error out when the private
Qt5 files are not found among the cached versions.

CMakeLists.txt

@@ -60,6 +60,9 @@ if (Qt5Sql_FOUND)
 		message(STATUS "Using cached/shipped versions of required header/source files.")
 		set(_PRIVATE_SOURCES_DIRECTORY "${PROJECT_SOURCE_DIR}/qt-file-cache/${Qt5Sql_VERSION}")
 		include_directories("${PROJECT_SOURCE_DIR}/qt-file-cache/${Qt5Sql_VERSION}")
+	elseif (CMAKE_BUILD_TYPE MATCHES "^Gentoo")
+		# No network access during Gentoo package build
+		message(SEND_ERROR "Header/source files for Qt5 version ${Qt5Sql_VERSION} not found. Please report this error!")
 	else()
 		if (Qt5Sql_VERSION VERSION_LESS 5.8.0)
 			set(DOWNLOAD_PATH "src/sql/drivers/sqlite")