That works, but it leaves a bunch of files owned by root in ./local/share/swig/3.0.9. 

Since you are installing it into a subdirectory, you don't actually need the sudo in install-swig.sh:

diff --git a/install-swig.sh b/install-swig.sh
index 07852f1c..0f9e0559 100755
--- a/install-swig.sh
+++ b/install-swig.sh
@@ -3,4 +3,4 @@ which sudo || alias sudo=eval
 which nproc || alias nproc="echo 1"

 git clone https://github.com/GeraldWodni/swig.git
-(cd swig && ./autogen.sh && ./configure "$@" && make -j`nproc` && sudo make install)
+(cd swig && ./autogen.sh && ./configure "$@" && make -j`nproc` &&  make install)

The script install-dep.sh installs swig in /usr/local, overwriting whatever is there.

Some packaging systems, notably brew at least, install all their files in /usr/local, and it's a common place for locally built packages to be installed.  Overwriting something installed by the user is a bad idea.

It should instead be installed as a subdirectory of the build directory, and either PATH should be modified, or explicit paths to executables should be used.