Ports: Add a cmake port

Yes, you build this *inside* serenity.

Ports/cmake/howto.md

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+## Build Guide
+
+- Run `./package.sh` here, which should install the dependencies and copy the patched source code to `~anon/Source/cmake`
+- Give the vm at least 2G of RAM, and at least 1.5G of free disk space (actual values used may be different, but ~1.3GiB of RAM and 1.2GiB of disk space has been observed)
+- Build the bootstrap cmake binary:
+```sh
+$ cd Source/cmake
+$ ./bootstrap
+```
+- Go entertain yourself for a few minutes (build takes about 10m)
+- The bootstrap binary should be built, but may fail for any of the reasons detailed in [Troubleshooting](#troubleshooting).
+- Build and install cmake (this takes a long time, about 80 minutes on my machine)
+```sh
+$ make
+# mount -o bind /usr /usr
+# make install
+```
+- Optionally, add `/usr/local/bin` to PATH:
+```sh
+$ export PATH="$PATH:/usr/local/bin"
+```
+- Hopefully look back at the experience and laugh at its ease
+
+## Current Status
+
+Fully working :P
+
+## Troubleshooting
+
+### "Failed to open check cache file for write..." while bootstrap cmake is configuring cmake
+
+The cause for this is unknown, but it seems to be a transitive state; simply restarting the bootstrap process fixes it:
+```sh
+$ Bootstrap.cmk/cmake . -C Bootstrap.cmk/InitialCacheFlags.cmake -G 'Unix Makefiles' -DCMAKE_BOOTSTRAP=1 -DBUILD_TESTING=0
+```
+You might have to repeat this many times.
+
+### "unable to rename '...', reason: File already exists" while making cmake
+
+Cause unknown. re-running make made it go away.