Visual Studio Open Folder GCC (Cygwin) Intellisense Errors - c

Question

I'm trying to configure Visual Studio Community 2019 for use as the IDE for work on an open source C project targeting the Nintendo 64.
The makefile seems to define a gcc-derivative (mips64-elf-gcc) as the compiler and has -std=gnu99 as one of the CFLAGS, so I assume that means this is C99.
I don't know much about C development, so I initially followed a tutorial, which directed me to set up Cygwin. For a time I edited the *.h and *.c files in Notepad++ and then used the Cygwin terminal to compile everything (there's probably a thousand better ways, but I'm a C/*nix nub and just wanted to get something off the ground quickly). Notepad++ was ok just to tinker around with things, but I'm getting more serious about this project and doing any real work in Notepad++ is a real bother. I've used Visual Studio for a long time, so I figured I'd try getting things set up there, since I know it has support for C/C++.
I've successfully gotten VS to run my makefile using the "Open Folder" feature and by wiring up the requisite data within the CppProperties.json and tasks.vs.json files. While attempting to get IntelliSense working, however, I've run into a few errors that I can't resolve.
1. 'duplicate parameter name' in stdio.h & string.h
There are several instances of this error. The problem seems to be lines like this one:
FILE * freopen (const char *__restrict, const char *__restrict, FILE *__restrict);
As you can see, there are multiple "parameters" all named __restrict. I'm still just beginning my C journey, so I'm not entirely sure what this is, but my suspicion was that it wasn't a parameter name, but might be #defineed somehwere. Using the Agent Ransack file search utility, I looked for instances of #define __restrict and did find some. I added those paths to the INCLUDE section of my CppProperties.json file, but that didn't help at all.
According to the VS Open Folder tutorial, IntelliSense can sometimes get hung up on preprocessor directives, so I added the recommended cpp.hint file, closed VS, deleted cached data in the .vs folder, and started VS back up, but the errors still persist.
2. "expected a ')'" in stdio.h
I assume this one is also related to that __restrict issue.
FILE * fopen (const char *__restrict _name, const char *__restrict _type);
^
| expected a ')'
It seems to be unhappy about the _name parameter for some reason.
3. 'expected an expression / identifier "i" is undefined' in my for loops
Any place in the code I've been working on where a for loop is defined, I get these errors. Here's an example of the errors:
void v(void)
{
for (int i = 0; i < 25; i++)
{ ^ ^
| | identifier "i" is undefined
|
| expected an expression
}
}
If, however, I change the loop to the following, everything is fine.
void v(void)
{
int i;
for (i = 0; i < 25; i++)
{
}
}
It's not liking the int type declaration in in the init-expression of the for loop. I'm not sure what I could've done in configuration to break a core language feature. I assume something about the previous errors are affecting this.
None of this affects the build process, since all of that is still being done through the makefile using everything I have set up in Cygwin, so I guess it isn't critical to fix these issues, but the whole point of doing this was to have a nice environment to work in and having 61 errors constantly hanging out is probably gonna put a pretty big damper on that situation.
I've tried everything I can think of to resolve these issues and nothing has worked. This is a particularly difficult issue to google, due to the large number of potential contributing factors required as search terms, so that hasn't proved fruitful.
Any help or advice is appreciated.