[LyX/master] Try with Qt4 when configuring for Qt5 fails

[Enrico Forestieri]

commit 0cab640035af3defda1edb796c20bd6203f7af46
Author: Enrico Forestieri <for...@lyx.org>
Date:   Mon Aug 27 16:31:51 2018 +0200

    Try with Qt4 when configuring for Qt5 fails
    
    This allows to avoid using --disable-qt5 when only the Qt4 libraries
    are available.
---
 config/qt4.m4 |   25 ++++++++++++++++++++++---
 1 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/config/qt4.m4 b/config/qt4.m4
index f53f243..75abd33 100644
--- a/config/qt4.m4
+++ b/config/qt4.m4
@@ -56,8 +56,12 @@ AC_DEFUN([QT_CHECK_COMPILE],
        ])
 
        if test -z "$qt_cv_libname"; then
-               AC_MSG_RESULT([failed])
-               AC_MSG_ERROR([cannot compile a simple Qt executable. Check you 
have the right \$QTDIR.])
+               if test x$USE_QT5 = xyes ; then
+                       AC_MSG_RESULT([failed, retrying with Qt4])
+               else
+                       AC_MSG_RESULT([failed])
+                       AC_MSG_ERROR([cannot compile a simple Qt executable. 
Check you have the right \$QTDIR.])
+               fi
        else
                AC_MSG_RESULT([$qt_cv_libname])
        fi
@@ -170,7 +174,22 @@ AC_DEFUN([QT_DO_IT_ALL],
        fi
 
        if test -z "$QT_LIB"; then
-         AC_MSG_ERROR([cannot find qt libraries.])
+         dnl Try again with Qt4 if configuring for Qt5 fails
+         if test x$USE_QT5 = xyes ; then
+               USE_QT5=no
+               AC_SUBST([USE_QT5])
+               if test -n "$PKG_CONFIG" ; then
+                 QT_DO_PKG_CONFIG
+               fi
+               if test "$pkg_failed" != "no" ; then
+                 QT_DO_MANUAL_CONFIG
+               fi
+               if test -z "$QT_LIB"; then
+                 AC_MSG_ERROR([cannot find qt libraries.])
+               fi
+         else
+               AC_MSG_ERROR([cannot find qt libraries.])
+         fi
        fi
 
        dnl Check qt version