Updates to build on Ubuntu 18.04 LTS (…)

- replace `add_compile_definitions` (added in CMake ≥3.12) with
`add_definitions` in `CMakeLists.txt`

    * hat tip to: https://stackoverflow.com/a/61250302/785213

- add #ifdef's to support Qt 5.9.5 for Ubuntu 18.04 by replacing QString
`.front()` and `.back()` methods with `.at(0)` and `.at(s.size() - 1)`

    * hat tip to: https://github.com/pawelsalawa/sqlitestudio/commit/90ca398

- add #ifdef's to replace `QFontMetrics` `.horizontalAdvance()` method
with `.width()` to support Qt 5.9.5 on Ubuntu 18.04

    * hat tip to: https://github.com/Sigil-Ebook/Sigil/commit/a32f890

- closes #107

src/SeerPrintpointCreateDialog.cpp

@@ -218,13 +218,21 @@ QString SeerPrintpointCreateDialog::printpointText () const {
     // Build the format string, ensuring a \" at the beggining and end of the string.
     printpointParameters += " ";
 
+#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
     if (format().front() != "\"") {
+#else
+    if (format().at(0) != "\"") {
+#endif
         printpointParameters += "\"";
     }
 
     printpointParameters += format();
 
+#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
     if (format().back() != "\"") {
+#else
+    if (format().at(format().size() - 1) != "\"") {
+#endif
         printpointParameters += "\"";
     }