gcc: Fix the conflicts between the production and devel ports

[cjones051073]

Description

Fixes some issues with the conflicts between the pro and devel gcc ports.

See https://trac.macports.org/ticket/57384

Type(s)

• bugfix
• enhancement
• security fix

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?

• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?

[ryandesign]

Thanks, this seems right.

[macportsbot]

Travis Build #3966 Errored.

Lint results

--->  Verifying Portfile for gcc7
--->  0 errors and 0 warnings found.
--->  Verifying Portfile for gcc8
--->  0 errors and 0 warnings found.
--->  Verifying Portfile for gcc9
--->  0 errors and 0 warnings found.
--->  Verifying Portfile for libgcc
--->  0 errors and 0 warnings found.

Port gcc7 success on xcode9.4. Log
Port libgcc7 success on xcode9.4. Log
Port gcc8 success on xcode9.4. Log
Port libgcc8 success on xcode9.4. Log
Port gcc9 success on xcode9.4. Log
Port libgcc-devel success on xcode9.4. Log
Port libgcc success on xcode9.4. Log
Port gcc7 success on xcode7.3. Log
Port libgcc7 success on xcode7.3. Log
Port gcc8 success on xcode7.3. Log
Port libgcc8 success on xcode7.3. Log
Port gcc9 success on xcode7.3. Log
Port libgcc-devel success on xcode7.3. Log
Port libgcc success on xcode7.3. Log
Port gcc7 success on xcode10. Log
Port libgcc7 success on xcode10. Log
Port gcc8 success on xcode10. Log
Port libgcc8 success on xcode10. Log
Port gcc9 success on xcode10. Log
Port libgcc-devel success on xcode10. Log
Port libgcc success on xcode10. Log