Some open source projects explicitly state that in order to compile on windows, they need a microsoft compiler (often a specific version as well, as latter versions are incompatible or will refuse to compile older code).
Since it seems absurd to me that, since there are foss compilers that can compile for windows, a microsoft compiler would be necessary for any fundamental task, I'm assuming this is because those projects use api calls to libraries (such as msvcrt*.dll) that, for some reason, mingw-gcc, clang and other ports of compilers for windows are unable to compile against.
My understanding of these requirements is shallow, since my experience with compiled code comes primarily from linux and this worries me, since getting a microsoft compiler is non-trivial. the only way to get them is through the express editions of microsoft's visual c++, and even then, the most recent version will completely refuse to install on an old winxp machine like mine and the only version available at the moment is vc++express2010, which requires registrations to turn from trialware into freeware (and even then i'm not clear on if that'll work or what it entails - perhaps OS hooks to "debug" and other intereference?).
1) My question is, do these projects depend on microsoft compilers due to building against these microsoft-only libraries (which apparently foss compilers can't do)?
It would seem absurd if the reason is the build script or preprocessor directives, since those can be relatively easily ported.
2) Also, is it possible that, even if I avoid any msvcrt/.net/etc. calls, i can still find myself needing a microsoft compiler to compile native windows software (assuming no usage of libraries that do perform those calls)?
3) Can I simply use clang and some widget library to make native windows software just as well?
4) Can I modify the source of a project so that it doesn't depend on a microsoft compiler?
(ok that's 4 questions, sorry, this is quite hard for me to express clearly).
1) My question is, do these projects depend on microsoft compilers due to building against these microsoft-only libraries (which apparently foss compilers can't do)?
Compiler vendors and GUI framework vendors can supply DLLs that perform similar to the MS DLLs. Some of the MS DLLs are system DLLs and are used by the other compiler and framework vendors.
If you are using compiler or framework specific DLLs, they need to accompany the installation of your programs (projects).
2) Also, is it possible that, even if I avoid any msvcrt/.net/etc. calls, i can still find myself needing a microsoft compiler to compile native windows software (assuming no usage of libraries that do perform those calls)?
No. If you scan through the posts on StackOverflow, there are many people who are using the Windows API directly, I guess what you are calling native windows software. Usually, the code for these API are located in a system API. The compiler translates the function call to a call into these DLLs, loading them as necessary.
3) Can I simply use clang and some widget library to make native windows software just as well?
No, you can't. That's why they exist. Again, many people are using frameworks like Qt and xWidgets without the MS compilers. I did that for a while. I switched over to Visual Studio, primarily for the debugger. I didn't like how other IDEs tried to use GDB. Otherwise, I wouldn't use MS because they tend to go by the Microsoft Standard language rather than the ISO.
4) Can I modify the source of a project so that it doesn't depend on a microsoft compiler?
No, that is why there are freeware and other compilers out there.
Hmmm, one can use Java to create GUIs that don't use the MS compiler, but they use the Windows API.
Try installing Cygwin. When you look at all the libraries you will realize that projects can be created that don't use the MS Compiler. Again, read through the StackOverflow posts and you will find that people are using other compilers, such as Intel, GNU, Clang, Greenhills and others. Some compilers for embedded systems will also compile for Windows OS, so you can write code that works on both platforms.
Looks like you need to search the web for "GNU GUI tutorial C++" and see what pops up. Also, search for "wxWidgets" and "Qt" for other frameworks.