added README for MacOS users. Thanks to Joao Roberto for testing and reporting instructions.

README_MacOS.md

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+# Compilation Instructions for xschem on MacOS
+In order to compile xschem properly on MacOS, we always need to reference the X libraries from XQuartz
+and change some compilation variables. The following dependencies are required:
+- XQuartz: https://www.xquartz.org/releases/XQuartz-2.7.11.html
+- Tcl: https://prdownloads.sourceforge.net/tcl/tcl8.6.10-src.tar.gz
+- Tk: https://prdownloads.sourceforge.net/tcl/tk8.6.10-src.tar.gz
+The first step is to install XQuartz and then compile Tcl and Tk. Nothing special on compiling these two, but
+we need to specify the X libraries from XQuartz when compiling Tk. Let's use the path
+/usr/local/opt/tcl-tk as destination folder.
+## Tcl compilation
+Extract the Tcl sources and then go to the unix folder:
+
+cd <extracted-folder>/unix
+./configure --prefix=/usr/local/opt/tcl-tk
+make
+make install
+Tk compilati on
+Same procedure as Tcl, but we need to specificy the Tcl and X libraries paths. XQuartz is installed on
+/opt/X11 , so we do:
+cd <extracted-folder>/unix
+./configure --prefix=/usr/local/opt/tcl-tk --with-tcl=/usr/local/opt/tcl-
+tk/lib --with-x --x-includes=/opt/X11/include --x-libraries=/opt/X11/lib
+make
+make install
+xschem compilati on
+Besides referencing the X libraries, we need to also point to the Tcl/Tk installation path. Let's use a recent
+xschem repository and install it on ~/xschem-macos (adapt this to your username):
+git clone https://github.com/StefanSchippers/xschem.git
+cd xschem
+./configure --prefix=/Users/$(whoami)/xschem-macos
+1 / 2README.md
+12/8/2020
+Before running compile the application, we need to adjust Makefile.conf because the current configure
+script doesn't support custom flags. So we need to replace the CFLAGS and LDFLAGS variables in that file
+as below:
+CFLAGS=-I/opt/X11/include -I/opt/X11/include/cairo -I/usr/local/opt/tcl-
+tk/include -O2
+LDFLAGS=-L/opt/X11/lib -L/usr/local/opt/tcl-tk/lib -lm -lcairo -lX11 -
+lXrender -lxcb -lxcb-render -lX11-xcb -lXpm -ltcl8.6 -ltk8.6
+Finally, we compile and install the application.
+make
+make install
+The application will be placed in /Users/$(whoami)/xschem-macos/bin and can be started with
+./xschem in that folder.