Missing declarations of long complex double transcendental functions in complex.h

[Ian Abbott <abbotti@mev.co.uk>]

This problem came up on stackoverflow.com in a question about using 
cos() from <tgmath.h> using newlib 3.3.0 and arm-none-eabi-gcc 9.2.1.

See https://stackoverflow.com/q/63632989/5264491

test.c:

#include <tgmath.h>

void test()
{
     double x=cos(4.5);
}

$ arm-none-eabi-gcc -c test.c
In file included from test.c:1:
test.c: In function 'test':
test.c:5:14: error: 'ccosl' undeclared (first use in this function); did 
you mean 'ccosh'?
     5 |     double x=cos(4.5);
       |              ^~~
test.c:5:14: note: each undeclared identifier is reported only once for 
each function it appears in
test.c:5:14: error: argument 6 of '__builtin_tgmath' is not a function 
pointer

It seems to be because various functions in <complex.h> are only 
declared if the '__CYGWIN__' macro is defined.  Note that for 
arm-none-eabi, the '_LDBL_EQ_DBL' macro is defined in <newlib.h>.

Best regards,
Ian Abbott

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