Add test case for gnumakefile detection

test/test_c_flag_parsing.vader

@@ -38,6 +38,14 @@ Execute(The make command should be correct):
   \   . 'make -n',
   \ ale#c#GetMakeCommand(bufnr(''))
 
+Execute(Should recognize GNUmakefile as a makefile):
+  call ale#test#SetFilename('test_c_projects/gnumakefile_project/file.c')
+
+  AssertEqual
+  \ ale#path#CdString(ale#path#Simplify(g:dir. '/test_c_projects/gnumakefile_project'))
+  \   . 'make -n --always-make',
+  \ ale#c#GetMakeCommand(bufnr(''))
+
 Execute(The CFlags parser should be able to parse include directives):
   call ale#test#SetFilename('test_c_projects/makefile_project/subdir/file.c')