...fltk-1.3.6rc1/test$ mv blocks.app/ blocks_original.app
...fltk-1.3.6rc1/test$ tree blocks_original.app/
blocks_original.app/
└── Contents
    ├── Info.plist
    ├── PkgInfo
    └── Resources
        └── blocks.icns

2 directories, 3 files

...fltk-1.3.6rc1/test$ mkdir -p blocks.app/Contents/Resources
...fltk-1.3.6rc1/test$ code blocks.app/Contents/Info.plist
...fltk-1.3.6rc1/test$ file blocks_original.app/Contents/PkgInfo 
blocks_original.app/Contents/PkgInfo: ASCII text
...fltk-1.3.6rc1/test$ cat blocks_original.app/Contents/PkgInfo 
BLKSBlks
...fltk-1.3.6rc1/test$ code blocks.app/Contents/PkgInfo 
...fltk-1.3.6rc1/test$ cp blocks_original.app/Contents/Resources/blocks.icns blocks.app/Contents/Resources/
...fltk-1.3.6rc1/test$ tree blocks.app/
blocks.app/
└── Contents
    ├── Info.plist
    ├── PkgInfo
    └── Resources
        └── blocks.icns

2 directories, 3 files

...fltk-1.3.6rc1$ make -j7

WORKS!