https://developers.redhat.com/blog/2019/04/22/implicit-function-declarations-flexs-use-of-reallocarray
Implicit function declarations: flex's use of "reallocarray" | Red Hat Developer
Apr 17, 2019 - Compatibility with existing build systems is improved if "reallocarray" is declared under _DEFAULT_SOURCE in Fedora's flex package.
flex sred hatimplicitfunctiondeclarations