`-o` does not work for relative `.g4` paths

[jirislaby]

> mkdir -p a_dir/build
> cd a_dir
> cat > A.g4 <<EOF
grammar A;

cmd : 'a' ;
EOF
> cd build
> java -classpath /usr/share/java/antlr4/antlr4.jar:/usr/share/java/antlr3-runtime.jar:/usr/share/java/antlr4/antlr4-runtime.jar:/usr/share/java/stringtemplate4/ST4.jar:/usr/share/java/treelayout/org.abego.treelayout.core.jar org.antlr.v4.Tool -o here ../A.g4
context [/report CANNOT_WRITE_FILE] 1:19 attribute arg isn't defined
error(1):  cannot write file : 
error(1):  cannot write file AParser.java: 
error(1):  cannot write file A.tokens: 

antlr apparently does not handle ../ paths correctly.

This is a problem in build systems as for example in meson, build dir have to be used and antlr is supposed to take the file from srcdir. Currently, I have to first cp the grammar to build dir:

antlr4 = find_program('antlr4')

copy_grammar = custom_target('copy_grammar',
  input : 'Make.g4',
  output : 'Make.g4',
  command : [ 'cp', '@INPUT@', '@OUTPUT@' ]
)

gen_antlr = custom_target('gen_antlr',
  input : copy_grammar,
  output : generated_files,
  command : [ antlr4, '-Dlanguage=Cpp', '-message-format', 'gnu', '@INPUT@' ],
)