gcc on mojave miss the system includes to /usr/local/include

[howard0su]

• are reporting a bug others will be able to reproduce and not asking a question. If you're not sure or want to ask a question do so on our Discourse: https://discourse.brew.sh
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• ran brew update and can still reproduce the problem?
• ran brew doctor, fixed all issues and can still reproduce the problem?
  MacBook-Air:src howardsu$ brew doctor
  Please note that these warnings are just used to help the Homebrew maintainers
  with debugging if you file an issue. If everything you use Homebrew for is
  working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: You have external commands with conflicting names.
Found command brew-aspell-dictionaries in following places:
/usr/local/Homebrew/Library/Taps/gtdmmb/homebrew-core/cmd/brew-aspell-dictionaries.rb
/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/cmd/brew-aspell-dictionaries.rb
Found command brew-postgresql-upgrade-database in following places:
/usr/local/Homebrew/Library/Taps/gtdmmb/homebrew-core/cmd/brew-postgresql-upgrade-database.rb
/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/cmd/brew-postgresql-upgrade-database.rb

• ran brew gist-logs <formula> (where <formula> is the name of the formula that failed) and included the output link?
• if brew gist-logs didn't work: ran brew config and brew doctor and included their output with your issue?

MacBook-Air:pages howardsu$ brew config
HOMEBREW_VERSION: 1.8.5
ORIGIN: https://github.com/Homebrew/brew
HEAD: 38493a3
Last commit: 10 days ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: 497ca90
Core tap last commit: 10 hours ago
HOMEBREW_PREFIX: /usr/local
CPU: quad-core 64-bit sandybridge
Homebrew Ruby: 2.3.7 => /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby
Clang: 10.0 build 1000
Git: 2.20.1 => /usr/local/bin/git
Curl: 7.54.0 => /usr/bin/curl
Java: 1.7.0_71
macOS: 10.14.2-x86_64
CLT: 10.1.0.0.1.1539992718
Xcode: N/A
XQuartz: 2.7.11 => /opt/X11

To help us debug your issue please explain:

• What you were trying to do (and why)
  Compile code under gcc with some local installed brew package. The issue is the new compiler doesn't include /usr/local/include as a system include folder.

• What happened (include command output)
  target/common/emu/fltk.cpp:28:10: fatal error: FL/Fl.H: No such file or directory
  #include <FL/Fl.H>
  ^~~~~~~~~
  compilation terminated.

• What you expected to happen
  successfully compile the code

• Step-by-step reproduction instructions (by running brew install commands)
  brew install gcc
  brew install fltk
  compile the following code:
  // a.c
  #include <FL/Fl.H>

void main() { return; }

[MikeMcQuaid]

• ran brew config and brew doctor and included their output with your issue?

@howard0su Please provide this output.

CC @fxcoudert for clarification on the expected behaviour here.

[howard0su]

provided all details.

[fxcoudert]

clang header search path on Mojave is:

 /usr/local/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/10.0.0/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
 /usr/include
 /System/Library/Frameworks (framework directory)
 /Library/Frameworks (framework directory)

Homebrew's GCC 8 is:

 /usr/local/Cellar/gcc/8.2.0/lib/gcc/8/gcc/x86_64-apple-darwin18.0.0/8.2.0/include
 /usr/local/Cellar/gcc/8.2.0/lib/gcc/8/gcc/x86_64-apple-darwin18.0.0/8.2.0/include-fixed
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks

so I feel like this is "sort of" a bug.

It tries to look up /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include, but this directory does not exist. We could try to configure GCC with --with-local-prefix, but I am not sure if this plays well with our use of --with-sysroot or not.

[MikeMcQuaid]

so I feel like this is "sort of" a bug.
We could try to configure GCC with --with-local-prefix

@fxcoudert Feel free to close out or open/request a PR; your call.

[MikeMcQuaid]

@fxcoudert Oh, and: thanks! ❤️