MuPDF is designed to compile and run on almost any platform that supports a standard C runtime library. In general, we try to restrict our use of C features later than C89, and to rely only on Posix standard APIs.
Our primary development platforms are Linux (various versions, 32 and 64bit using both Clang and GCC compilers) and Windows (32 and 64bit, using MSVC 2019).
The choice of MSVC 2019 was forced upon us by the Tesseract library that we use for OCR requiring a later version of C++ than is supported by earlier versions of Visual C. Executables can be produced by this version of Visual Studio that run on every Windows version from XP (Service Pack 3) up to the latest (Windows 10 20H2, at the time of writing). If Tesseract functionality is not required, then it should be fairly simple to fall back to earlier versions of VS (certainly back to VS2010, and even back to VS2005 at the cost of a different solution format).
Windows is not Posix compliant, but in the cases where a Posix API is required and not available, we provide windows specific code, often by implementing an equivalent API using Windows platform calls.
We do not currently actively use MacOS for developing MuPDF, though we have in the past. MuPDF compiles and runs fine on MacOS, and we intend to keep it so. There is no MacOS specific code in the MuPDF codebase; the MuPDF viewer for MacOS is just the standard linux X11 one.
Aside from the native C API, MuPDF has language bindings for C++, Python, Java and JavaScript.