Auto merge of #21540 - AgustinCB:fix-readme-build-instructions, r=jdm

Fix setup instructions for Arch

base-devel contains the last version of autoconf. However, some
dependencies of the project rely on autoconf 2.13. Without this change,
the build fails with the following message:

```
ERROR: Could not find autoconf 2.13

--- stderr
ERROR: Could not find autoconf 2.13
make: *** [makefile.cargo:143: maybe-configure] Error 1
thread 'main' panicked at 'assertion failed: result.success()', build.rs:110:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.
```

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---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they just change documentation on how to build.

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README.md

@@ -138,7 +138,7 @@ sudo zypper install libX11-devel libexpat-devel libbz2-devel Mesa-libEGL-devel M
 
 ``` sh
 sudo pacman -S --needed base-devel git python2 python2-virtualenv python2-pip mesa cmake bzip2 libxmu glu \
-    pkg-config ttf-fira-sans harfbuzz ccache clang 
+    pkg-config ttf-fira-sans harfbuzz ccache clang autoconf2.13
 ```
 #### On Gentoo Linux