I added the following to my jenkinsfile:
stage ('Warnings gcc')

{ step([$class: 'WarningsPublisher', canComputeNew: false, canResolveRelativePaths: false, defaultEncoding: '', excludePattern: '', healthy: '', includePattern: '', messagesPattern: '', parserConfigurations: [[parserName: 'GNU Make + GNU C Compiler (gcc)', pattern: 'error_and_warnings.txt']], unHealthy: '']) }

stage ('Warnings clang')

{ step([$class: 'WarningsPublisher', canComputeNew: false, canResolveRelativePaths: false, defaultEncoding: '', excludePattern: '', healthy: '', includePattern: '', messagesPattern: '', parserConfigurations: [[parserName: 'Clang (LLVM based)', pattern: 'error_and_warnings_clang.txt']], unHealthy: '']) }

And get the logs ( Stage logs ):
[WARNINGS] Parsing warnings in files 'error_and_warnings.txt' with parser GNU Make + GNU C Compiler (gcc)
[WARNINGS] Finding all files that match the pattern error_and_warnings.txt
[WARNINGS] Parsing 1 file in /home/jenkins/workspace/multibrach1_new_branch_001-BS43Z4K6ZIS5GYHNACODUZICSSNFL6N7QBEVH4WYM3M67DI7ZWWQ
[WARNINGS] Successfully parsed file /home/jenkins/workspace/multibrach1_new_branch_001-BS43Z4K6ZIS5GYHNACODUZICSSNFL6N7QBEVH4WYM3M67DI7ZWWQ/error_and_warnings.txt with 0 unique warnings and 0 duplicates.
[WARNINGS] Computing warning deltas based on reference build #1

and for clang:
[WARNINGS] Parsing warnings in files 'error_and_warnings_clang.txt' with parser Clang (LLVM based)
[WARNINGS] Finding all files that match the pattern error_and_warnings_clang.txt
[WARNINGS] Parsing 1 file in /home/jenkins/workspace/multibrach1_new_branch_001-BS43Z4K6ZIS5GYHNACODUZICSSNFL6N7QBEVH4WYM3M67DI7ZWWQ
[WARNINGS] Successfully parsed file /home/jenkins/workspace/multibrach1_new_branch_001-BS43Z4K6ZIS5GYHNACODUZICSSNFL6N7QBEVH4WYM3M67DI7ZWWQ/error_and_warnings_clang.txt with 3 unique warnings and 0 duplicates.
[WARNINGS] Computing warning deltas based on reference build #1

Looks ok for me, but I see no results! There is no graph displayed! The graph for unit test results ( gtest ) works in this view!