Makefile : contains string

The findstring function is what your heart desires:

$(findstring find,in)

Searches in for an occurrence of find. If it occurs, the value is find; otherwise, the value is empty. You can use this function in a conditional to test for the presence of a specific substring in a given string. Thus, the two examples,

$(findstring a,a b c)
$(findstring a,b c)

produce the values "a" and "" (the empty string), respectively. See Testing Flags, for a practical application of findstring.

Something like:

ifneq (,$(findstring NT-5.1,$(VARIABLE)))
    # Found
else
    # Not found
endif

What is the comma here for ifneq (,$(...?

Parse it as ifneq(A,B) where A is the empty string and B is $(findstring...). It looks odd because you don’t quote strings in Makefiles.