Update build instructions in README.Unix.txt (#67)

These build instructions are as up-to-date as possible although
there are still references to FLTK 1.3 (e.g. under NetBSD).

Closes #67.

README.Unix.txt

@@ -36,7 +36,7 @@ README.Unix.txt - Building FLTK on Unix/Linux Systems
   Note: usage of autotools, configure, and the included Makefiles
   to build the FLTK library is deprecated since FLTK 1.4 and will
   be removed in the next minor version (1.5).
-  Please consider using CMake instead.
+  Please consider using CMake instead, see README.CMake.txt.
 -------------------------------------------------------------------
 
 
@@ -73,7 +73,7 @@ See https://www.fltk.org/bugs.php for how to do this.
 All Linux distributions based on Debian are similar and use the `apt`
 package manager.
 
-  - Ubuntu Linux can be downloaded from https://ubuntu.com/
+  - Ubuntu Linux can be downloaded from https://ubuntu.com/download/desktop
   - Debian Linux can be downloaded from https://www.debian.org/
   - Linux Mint   can be downloaded from https://www.linuxmint.com/
 
@@ -167,7 +167,7 @@ and x*** (X Window System) are installed.
 
 Now install and configure pkgsrc. The current version can be downloaded here:
 
-  http://www.pkgsrc.org/
+  https://www.pkgsrc.org/
 
 To use the current stable version of FLTK, simply install it from pkgsrc:
 
@@ -291,11 +291,9 @@ dev directory. Change the following instructions to fit your preferences.
   cd
   mkdir dev
   cd dev
-  mv ~/Downloads/fltk-1.3.xxxx.tar.gz .
-  tar xvfz fltk-1.3.xxxx.tar.gz
-  cd fltk-1.3.xxxx
-
-Note: current development version is 1.4.0 (1.4.x).
+  mv ~/Downloads/fltk-1.x.y-source.tar.gz .
+  tar xvzf fltk-1.x.y-source.tar.gz
+  cd fltk-1.x.y
 
 
  3.2  Configuration and Build Systems
@@ -314,7 +312,7 @@ Makefiles, Ninja build files, Xcode or Visual Studio IDE projects etc..
   Note: usage of autotools, configure, and the included Makefiles
   to build the FLTK library is deprecated since FLTK 1.4 and will
   be removed in the next minor version (1.5).
-  Please consider using CMake instead.
+  Please consider using CMake instead, see README.CMake.txt.
 -------------------------------------------------------------------
 
 Please see README.CMake.txt for how to build FLTK and your application
@@ -335,7 +333,7 @@ be removed in FLTK 1.5.
   Note: usage of autotools, configure, and the included Makefiles
   to build the FLTK library is deprecated since FLTK 1.4 and will
   be removed in the next minor version (1.5).
-  Please consider using CMake instead.
+  Please consider using CMake instead, see README.CMake.txt.
 -------------------------------------------------------------------
 
 If you got FLTK via git then you need one extra step. Otherwise skip
@@ -428,12 +426,13 @@ source file is:
 
   fltk-config --compile myProgram.cxx
 
-`fltk-config --compile` can not be used to build a program from multiple source
-files but you can use it in a Makefile to get the required compiler and linker
-flags.
+Since version 1.4.0 `fltk-config --compile` can also be used to build a program
+from multiple source files. See the official docs in chapter "FLTK Basics" and
+section "Compiling Multiple Source Files with 'fltk-config'":
+https://www.fltk.org/doc-1.4/basics.html#basics_fltk_config2
 
-"fltk-config" and "fluid" will be installed in "/usr/local/bin/" by default. I
-recommend that you add it to the command search path.
+"fltk-config" and "fluid" will be installed in "/usr/local/bin/" by default.
+We recommend that you add it to the command search path.
 
 
 
@@ -443,8 +442,7 @@ recommend that you add it to the command search path.
 Code::Blocks is a free and popular C++ IDE in the Linux world. It also runs on
 macOS and Windows. Configured correctly, it can also cross-compile between
 these platforms. This chapter focuses on creating a new FLTK project for Linux,
-assuming that FLTK 1.3 was previously built and installed in its default
-location from the command line.
+assuming that FLTK was previously built and installed in its default location.
 
 If not done yet, install Code::Blocks as described in the "Prerequisites"
 chapter above, or download it from their web site. This description is based