Read the lines from the file[file2.txt] and concatenate the lines into single line then
search the string "INPUT=" from the another file[file1.txt] and if "INPUT" is present in that file[file1.txt]
assign the concatenate patterns into "INPUT=" pattern.

example:

file1.txt

Code:

<some lines>
INPUT = 
<some lines>

file2.txt

Code:

../A/B/sample.c
../A/c/sample1.c
../e/f/sample2.c
../d/B/sample3.c

Output:

INPUT= ../A/B/sample.c ../A/c/sample1.c ../e/f/sample2.c ../d/B/sample3.c