Optional build dependencies incomplete about glvis

[v-dobrev]

I think I've seen this issue before: #210.

We can add a check in the makefile with something like this:

XXD_BIN = $(or $(shell command -v xxd),$(error Required tool not found: xxd. Stop.))

I'm not sure what happens with CMake -- maybe it stops with an error if does not find xxd?

[termi-official]

Indeed, this is the same issue.

I would also have opted for checking if xxd is installed by using command, but with ?= instead of direct assignment.

"Fixing" the cmake build should be as easy as

find_program(XXD_BIN xxd)
if(NOT XXD_BIN)
    error(FATAL_ERROR "xxd not found!")
endif()

[termi-official]

Should I make a PR?

[v-dobrev]

Should I make a PR?

A PR will be welcome! Thanks!

[tzanio]

Discussion continued in @222