install: clang-11.x compatible PGO instructions

LLVM/Clang 11.x throws errors for _files_ without any profiling data:

  error: ../../async.c: Function control flow change detected (hash mismatch) async_write Hash = 72057641800614222 [-Werror,-Wbackend-plugin]

This caused multiple issues with the current PGO instructions:

* The `pgo` binary failed to link with `meson configure -Db_pgo=use`
  when doing a full PGO build (since it isn't used in this case);
* `footclient` fails to link for the same reason.
* `foot` fails to link for the same reason in **partial** PGO builds.

The solution is to make sure all binaries that are built in the final
phase (`-Db_pgo=use`) have been executed in the _generate_ phase.

Doing this also means we can drop `-Wno-missing-profile` and friends.

Also add `--sixel` to the `generate-alt-random-writes` command line in
the description for a full PGO build.

Closes #418

INSTALL.md

@@ -185,21 +185,15 @@ slower!) binary.
 First, configure the build directory:
 
 ```sh
-export CFLAGS="$CFLAGS -O3 -Wno-missing-profile"
+export CFLAGS="$CFLAGS -O3"
 meson --buildtype=release --prefix=/usr -Db_lto=true ../..
 ```
 
 It is **very** important `-O3` is being used here, as GCC-10.1.x and
 later have a regression where PGO with `-O2` is **much** slower.
 
-If you are using Clang instead of GCC, use the following `CFLAGS` instead:
-
-```sh
-export CFLAGS="$CFLAGS -O3 \
-    -Wno-ignored-optimization-argument \
-    -Wno-profile-instr-out-of-date \
-    -Wno-profile-instr-unprofiled"
-```
+Clang users **must** add `-Wno-ignored-optimization-argument` to
+`CFLAGS`.
 
 Then, tell meson we want to _generate_ profiling data, and build:
 
@@ -235,6 +229,8 @@ We will use the `pgo` binary along with input corpus generated by
 `scripts/generate-alt-random-writes.py`:
 
 ```sh
+./footclient --version
+./foot --version
 tmp_file=$(mktemp)
 ../../scripts/generate-alt-random-writes \
     --rows=67 \
@@ -254,7 +250,12 @@ tmp_file=$(mktemp)
 rm ${tmp_file}
 ```
 
-The snippet above first creates an (empty) temporary file. Then, it
+The first step, running `./foot --version` and `./footclient
+--version` might seem unnecessary, but is needed to ensure we have
+_some_ profiling data for functions not covered by the PGO helper
+binary. Without this, the final link phase will fail.
+
+The snippet above then creates an (empty) temporary file. Then, it
 runs a script that generates random escape sequences (if you cat
 `${tmp_file}` in a terminal, you’ll see random colored characters all
 over the screen). Finally, we feed the randomly generated escape
@@ -272,14 +273,19 @@ This method requires a running Wayland session.
 We will use the script `scripts/generate-alt-random-writes.py`:
 
 ```sh
+./footclient --version
 foot_tmp_file=$(mktemp)
-./foot --config=/dev/null --term=xterm sh -c "<path-to-generate-alt-random-writes.py> --scroll --scroll-region --colors-regular --colors-bright --colors-256 --colors-rgb --attr-bold --attr-italic --attr-underline ${foot_tmp_file} && cat ${foot_tmp_file}"
+./foot --config=/dev/null --term=xterm sh -c "<path-to-generate-alt-random-writes.py> --scroll --scroll-region --colors-regular --colors-bright --colors-256 --colors-rgb --attr-bold --attr-italic --attr-underline --sixel ${foot_tmp_file} && cat ${foot_tmp_file}"
 rm ${foot_tmp_file}
 ```
 
 You should see a foot window open up, with random colored text. The
 window should close after ~1-2s.
 
+The first step, `./footclient --version` might seem unnecessary, but
+is needed to ensure we have _some_ profiling data for
+`footclient`. Without this, the final link phase will fail.
+
 
 ##### Use the generated PGO data