create executables at end of CI pipeline

[slivingston]

The most frequent criticism that I receive about gr1c is that building gr1c is difficult on macOS and Windows. This can be practically solved by providing users with ready-to-execute files for macOS and Windows. To this end, I propose to extend the CI testing pipeline to save build results in certain cases: version tags or every commit on master branch.

Consider the following:

1. Travis CI provides for uploading assets in tagged releases on GitHub,
2. AppVeyor provides a similar feature and furthermore has Windows VMs.

[slivingston]

The mingw-w64 project (also, the project on SourceForge) provides GCC prepared as a cross-compiler that targets 32-bit and 64-bit Windows.

I am able to build gr1c with it, and I have tried several examples while executing gr1c.exe on Wine and on Windows 10.

[slivingston]

as part of this work, delete the files posted at https://dl.bintray.com/slivingston/generic/ and references to that in the manual

[slivingston]

I am using https://github.com/tpoechtrager/osxcross in other work, and I propose that it can be used here to build gr1c files for macOS hosts. Today on a GNU/Linux host (my laptop), I built files that present as macOS executables from the following steps:

git clone https://github.com/tpoechtrager/osxcross.git
cd osxcross
wget -nc https://s3.dockerproject.org/darwin/v2/MacOSX10.10.sdk.tar.xz
mv MacOSX10.10.sdk.tar.xz tarballs
UNATTENDED=yes OSX_VERSION_MIN=10.7 ./build.sh
export PATH=/home/scott/extern-repo/osxcross/target/bin:$PATH

Then, from the root of the gr1c source tree, after completion of ./get-deps.sh

export CC=x86_64-apple-darwin14-cc
export LD="x86_64-apple-darwin14-ld -r"
cd extern/src/cudd-3.0.0
make clean
CC=o64-clang CXX=o64-clang++ ./configure --host=x86_64-apple-darwin14 --prefix=`pwd`/../..
make
make install
cd ../../..
make -e all

[slivingston]

For completeness, the SHA256 value of the tarball downloaded above is as follows:

631b4144c6bf75bf7a4d480d685a9b5bda10ee8d03dbf0db829391e2ef858789  tarballs/MacOSX10.10.sdk.tar.xz

[slivingston]

N.B., using GitHub Actions as of cc46891