Documentation unclear about requirements / build.sh no good perceptible warnings

*This issue was originally created by [**@raypatterson77**](https://github.com/raypatterson77) at .*

Link to problem area: https://github.com/matrix-org/dendrite/blob/main/docs/INSTALL.md

# Issue
## Documentation is unclear about requirements

- Requirements are missing git and gcc
- For git it is very trivial to be a requirement due to cloning the repository
- But without gcc the build.sh script is not failing or displaying good perceptible warnings / errors.
- On a fresh installed Debian 11 (5.10.0-11-amd64) I was able to run the build.sh but had no generate-keys nor dendrite-monolith-server in the bin folder.
Running:
```bash
go build -o bin/generate-keys ./cmd/generate-keys
```
-gave me the error, that no gcc was found in $PATH
- after installing gcc:

```bash
gcc --version
gcc (Debian 10.2.1-6) 10.2.1 20210110
[...]
```
- and clone a new repository, running build.sh works as expected
- Unfortunately I not examine the gcc problem further, but as said, it was a fresh installation of Debian 11
```bash
/usr/bin
```
- was definitly in $PATH

## Expected behavior

- requirements should be clearer in documentation or the build.sh script should give more perceptible warnings / errors

If this is not wanted for a particular reason I can create a pull request and add git and gcc to the requirements but I'm not sure about the miniumum versions

Start with docs/INSTALL.md and build.sh, then reproduce the fresh Debian 11 setup described in the issue, checking the existing build output and generated bin files. Done means the documented requirements include git and gcc, or build.sh gives perceptible warnings or errors when prerequisites are absent.

Written by the indexing model from the issue text.