doc: update README.md and doc/MEX_INTERFACE.md

Author: siko1056

README.md

@@ -40,6 +40,9 @@ interface (explained below) and vectorization wherever possible.
 
 ## Low-level MPFR Interface
 
+> For information how to compile/develop the interface, see
+> [`doc/MEX_INTERFACE.md`](https://github.com/gnu-octave/apa/blob/main/doc/MEX_INTERFACE.md).
+
 The low-level MPFR interface permits efficient access to almost all functions
 specified by MPFR 4.1.0 <https://www.mpfr.org/mpfr-current/mpfr.html>.
 

doc/MEX_INTERFACE.md

@@ -44,14 +44,29 @@ The APA MEX interface is known to work and tested for
 
 > **MS Windows Note:** To compile MPFR with Visual Studio, please look at the
 > following project <https://github.com/BrianGladman/mpfr>.  However, this
-> approach is not supported by APA and requires some adaptions.
+> approach is not supported by APA and requires some adaptions of this project.
 
 
 ## Compiling static GMP and MPFR libraries for MS Windows, macOS, and UNIX (Linux)
 
-For macOS and UNIX/Linux systems
+> Usually this step is **not** necessary, as pre-compiled static library
+> versions (`libgmp.a`, `libmpfr.a`, `gmp.h`, `mpfr.h`, and `mpf2mpfr.h`)
+> are included in the released package in the respective folders
+> - `apa/inst/mex/macos`
+> - `apa/inst/mex/mswin`
+> - `apa/inst/mex/unix`
 
-<https://wiki.octave.org/Windows_Installer>
+For macOS and UNIX/Linux systems (Octave or Matlab) this can be conveniently
+done by calling:
+```matlab
+cd apa/inst/mex
+system ('make')
+```
+
+For MS Windows one has to cross-compile the respective libraries using
+the [octave-mxe](https://wiki.octave.org/Windows_Installer) project.
+
+> The following steps have to be done on Linux (e.g. in a virtual machine).
 
 ```
 hg clone https://hg.octave.org/mxe-octave
@@ -72,13 +87,3 @@ cp ./usr/x86_64-w64-mingw32/lib/libgmp.a \
    ./usr/x86_64-w64-mingw32/include/mpf2mpfr.h \
    /path/to/apa/inst/mex/mswin
 ```
-
-### Matlab
-
-> Only Matlab R2018b and later
-
-
-
-
-
-$(ENABLE_SHARED_OR_STATIC) --with-pic=yes