updated Scintilla to v5.3.4, Scinterm to v4.1 and Lexilla to v5.2.4

* actually everything is updated to their current HEADs but the aforementioned versions are close.
* Scintilla uses threads now, so we added checks for pthread.
  To be on the safe side, we imported AX_PTHREAD from the Autoconf archives.
  The flags are kept out of the ordinary build system, though and used only for compiling Scintilla
  and for linking.
  SciTECO may also use threads, but via Glib.
* Scinterm removed SCI_COLOR_PAIR(), so we re-added it to src/interface-curses/interface.c.
* There is an Asciidoc lexer now.
* The <Ix$> interruption bug (see TODO) is not fixed by this upgrade.
  Perhaps the Mac OS version runs better now. Feedback is needed (refs #12).

INSTALL

@@ -44,10 +44,10 @@ These dependencies are bundled with the SciTECO Git repository
 and with source tar balls, so they usually do not have to be
 installed by the user manually:
 
- * Scintilla (v5.0.0 or later):
+ * Scintilla (v5.3.0 or later):
    http://www.scintilla.org/
  * When choosing the Curses interface:
-   * Scinterm (v3.1 or later):
+   * Scinterm (v4.0 or later):
      http://foicica.com/scinterm/
  * Lexilla (v5.0.0 or later, optional):
    https://www.scintilla.org/Lexilla.html

configure.ac

@@ -134,6 +134,13 @@ PKG_CHECK_MODULES(LIBGLIB, [glib-2.0 >= 2.44], [
 	LIBS="$LIBS $LIBGLIB_LIBS"
 ])
 
+# Required by Scintilla.
+AX_PTHREAD([], [
+	AC_MSG_ERROR([pthread not found!])
+])
+SCINTILLA_CXXFLAGS="$SCINTILLA_CXXFLAGS $PTHREAD_CFLAGS"
+LIBS="$LIBS $PTHREAD_LIBS"
+
 case $host in
 *-mingw*)
 	AC_CHECK_HEADERS([windows.h], , [
@@ -200,11 +207,10 @@ AM_CONDITIONAL(LEXILLA, [test x$LEXILLA_PATH != xno])
 if [[ x$LEXILLA_PATH != xno ]]; then
 	LEXILLA_PATH=`canonicalize $LEXILLA_PATH`
 	AC_SUBST(LEXILLA_PATH)
+	CPPFLAGS="$CPPFLAGS -I$LEXILLA_PATH/include"
 	AC_DEFINE(HAVE_LEXILLA, 1, [Building with Lexilla (Lexer) support])
 fi
 
-CPPFLAGS="$CPPFLAGS -I$LEXILLA_PATH/include"
-
 AC_ARG_WITH(interface,
 	AS_HELP_STRING([--with-interface=ncurses|netbsd-curses|xcurses|pdcurses|pdcurses-gui|gtk],
 		       [Specify user interface [default=ncurses]]),