The xc32/xc16 GCC files are licensed under GPL3, which gives you the right to modify these files (source, binary- makes no difference). If you plan to also distribute the modified files (highly unlikely), then additional obligations are added.
Since it is next to impossible (for normal users) to compile the xc16/xc32 gcc compiler from sources MCHP provides, this is the next best thing- either modify the binary files to remove restrictions, or simply add a file and a global option to your project (no modifications needed).
(note- build scripts are now provided in xc32 v2.05 source, so maybe not impossible anymore)
If you do not want to modify a few binary files, then see Section 2 of the info file which is quite simple.
Here is the xc32 license (mchp.c)-
/* Subroutines used for Microchip PIC32 MCU support