gcc-avr preprocessor search path does not match gcc spec

[tomdean]

The search path for avr-cpp does not include /usr/local/include as in the gcc documents.

# cpp -v
...
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/9/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.

# avr-cpp -v
...
#include <...> search starts here:
 /usr/lib/gcc/avr/5.4.0/include
 /usr/lib/gcc/avr/5.4.0/include-fixed
 /usr/lib/gcc/avr/5.4.0/../../../avr/include
End of search list.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gcc-avr 1:5.4.0+Atmel3.6.1-2build1
ProcVersionSignature: Ubuntu 5.8.0-53.60~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-53-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.17
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: GNOME
Date: Tue May 25 06:14:40 2021
InstallationDate: Installed on 2021-05-22 (3 days ago)
InstallationMedia: Ubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gcc-avr
UpgradeStatus: No upgrade log present (probably fresh install)