Why does building the antlr4_runtime clone utfcpp?

[mithro]

[ 44%] Performing build_static step for 'antlr4_runtime'                                                                                          gmake[4]: Entering directory '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp'
gmake[5]: Entering directory '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp'
gmake[6]: Entering directory '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp'
gmake[7]: Entering directory '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp'               gmake[7]: Entering directory '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp'
Scanning dependencies of target make_lib_output_dir                                                                                               
Scanning dependencies of target utfcpp
gmake[7]: Leaving directory '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp'                gmake[7]: Leaving directory '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp'
gmake[7]: Entering directory '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp'
gmake[7]: Entering directory '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp'
gmake[7]: Leaving directory '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp'                [  0%] Creating directories for 'utfcpp'
[  0%] Built target make_lib_output_dir
[  1%] Performing download step (git clone) for 'utfcpp'
Cloning into 'utfcpp'...                                                                                                                          Note: switching to 'v3.1.1'.
                                    
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HEAD is now at 944ef05 Merge pull request #55 from Flamefire/disable_tests_for_subproject 
Submodule 'extern/gtest' (https://github.com/google/googletest) registered for path 'extern/gtest'
Cloning into '/ssd/github/SymbiFlow/fasm/build/temp.linux-x86_64-3.9/antlr4_runtime/src/antlr4_runtime/runtime/Cpp/runtime/thirdparty/utfcpp/exter
n/gtest'...
Submodule path 'extern/gtest': checked out '2fe3bd994b3189899d93f1d5a881e725e046fdc2'
[  1%] No patch step for 'utfcpp'
[  1%] Performing configure step for 'utfcpp'
-- The CXX compiler identification is GNU 10.2.1