This may not be approporiate place but ...

Cygwin creates a subdirectory /cygdrive where it stores phisical drives e.g. C: as c, F: as f, etc. Hence /cygdrive/f/CygWin/usr is a valid long name pointing to /usr directory in Cygwin

When used with Cygwin, MinGW gcc and ld do not follow paths given as "/cygdrive/f/CygWin/usr" when given as include or lib parths using -I or -L respectively though it does not complain either. But MinGW requires

     -I'F:/CygWin/usr' 

and similar for the -L option.

Hence...
If I tried to compile and build an application using MinGW gcc , it seems to ignore both, the include and library paths specified with
either CPATH, LD_LIBRARY_PATH or with individual options -I or -L
or -isystem. I.e. (and, e.g.) it can not find includes such as the pthread.h (or the libcatlas
libraries) that cygwin's default cc ( linked to the gcc) successfully finds, compiles and links
using the specified -I and -L options respectively.

george