From @dcollinsn

Greetings,

On Cygwin 2.5​:

$ uname -a
CYGWIN_NT-10.0 DESKTOP-DUOI7F4 2.5.2(0.297/5/3) 2016-06-23 14​:29 x86_64 Cygwin

Configure with​:

$ ./Configure -Dusedevel -Duselongdouble -des

Complains that​:

*** You requested the use of long doubles but you do not seem to have
*** the following mathematical functions needed for long double support​:
*** sqrtl modfl frexpl ldexpl
*** Please rerun Configure without -Duselongdouble and/or -Dusemorebits.
*** Cannot continue, aborting.

At least two libraries on my system provide sqrtl​: libm and libcygwin.

And in fact, configure has even already used this function. This test appears at Configure line 10345​:

: Before committing on uselongdouble, see whether that looks sane.
if $test "$uselongdouble" = "$define"; then
  message=""
  echo " "
  echo "Checking if your long double math functions work right..." >&4
  $cat > try.c <<EOF
#include <math.h>
#include <stdio.h>
int main() {
  printf("%"$sPRIgldbl"\n", sqrtl(logl(expl(cosl(sinl(0.0L))))+powl(2.0L, 3.0L)));
}
EOF
  case "$osname​:$gccversion" in
  aix​:) saveccflags="$ccflags"
  ccflags="$ccflags -qlongdouble" ;; # to avoid core dump
  esac
  set try
  if eval $compile_ok; then
  yyy=`$run ./try`
  fi
  case "$yyy" in
  3) echo "Your long double math functions are working correctly." >&4 ;;
  *) echo "Your long double math functions are broken, not using long doubles." >&4
  uselongdouble=$undef
  ;;
  esac
  $rm_try
  case "$osname​:$gccversion" in
  aix​:) ccflags="$saveccflags" ;; # restore
  esac
fi

And that test passed​:

Checking if your long double math functions work right...
Your long double math functions are working correctly.

Quite some time before Configure line 16436​:

sqrtl() NOT found.

In fact, the problem seems to be that libm is not included here​:

Extracting names from the following files for later perusal​:
  /usr/lib/libc.a
  /usr/lib/libdl.a
  /usr/lib/libpthread.a

Because hints/cygwin removed it from libswanted on account of it being a symlink to libcygwin.a. It's not, but even if it was, that isn't helpful because libcygwin.a /also/ isn't in libswanted. I added it, and Configure now is happy.

I suspect that isn't the /right/ fix, though. After all, a previous test in Configure has successfully linked against some library that provides sqrtl - how did that test work, if configure isn't aware of any lib that provides sqrtl?

--
Respectfully,
Dan Collins

%% perl -V when forced to use -lcygwin

$ ./perl -Ilib -V
Summary of my perl5 (revision 5 version 25 subversion 3) configuration​:
  Derived from​: f6203e9
  Platform​:
  osname=cygwin
  osvers=2.5.2(0.29753)
  archname=cygwin-thread-multi
  uname='cygwin_nt-10.0 desktop-duoi7f4 2.5.2(0.29753) 2016-06-23 14​:29 x86_64 cygwin '
  config_args='-Dusedevel -Duselongdoubles -des -Alibswanted=cygwin'
  hint=previous
  useposix=true
  d_sigaction=define
  useithreads=define
  usemultiplicity=define
  use64bitint=define
  use64bitall=define
  uselongdouble=undef
  usemymalloc=n
  bincompat5005=undef
  Compiler​:
  cc='gcc'
  ccflags ='-DPERL_USE_SAFE_PUTENV -U__STRICT_ANSI__ -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -D_FORTIFY_SOURCE=2'
  optimize='-O3'
  cppflags='-DPERL_USE_SAFE_PUTENV -U__STRICT_ANSI__ -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong'
  ccversion=''
  gccversion='5.4.0'
  gccosandvers=''
  intsize=4
  longsize=8
  ptrsize=8
  doublesize=8
  byteorder=12345678
  doublekind=3
  d_longlong=define
  longlongsize=8
  d_longdbl=define
  longdblsize=16
  longdblkind=3
  ivtype='long'
  ivsize=8
  nvtype='double'
  nvsize=8
  Off_t='off_t'
  lseeksize=8
  alignbytes=8
  prototype=define
  Linker and Libraries​:
  ld='g++'
  ldflags =' -Wl,--enable-auto-import -Wl,--export-all-symbols -Wl,--enable-auto-image-base -fstack-protector-strong -L/usr/local/lib'
  libpth=/usr/lib /usr/lib/gcc/x86_64-pc-cygwin/5.4.0/../../../../lib/../include/w32api /usr/local/lib /lib /usr/lib /usr/lib/gcc/x86_64-pc-cygwin/5.4.0/../../../../lib/../include/w32api
  libs=-lpthread -ldl -lcygwin
  perllibs=-lpthread -ldl -lcygwin
  libc=/usr/lib/libc.a
  so=dll
  useshrplib=true
  libperl=cygperl5_25_3.dll
  gnulibc_version=''
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs
  dlext=dll
  d_dlsymun=undef
  ccdlflags=' '
  cccdlflags=' '
  lddlflags=' --shared -Wl,--enable-auto-import -Wl,--export-all-symbols -Wl,--enable-auto-image-base -L/usr/local/lib -fstack-protector-strong'

Characteristics of this binary (from libperl)​:
  Compile-time options​:
  HAS_TIMES
  MULTIPLICITY
  PERLIO_LAYERS
  PERL_COPY_ON_WRITE
  PERL_DONT_CREATE_GVSV
  PERL_HASH_FUNC_ONE_AT_A_TIME_HARD
  PERL_IMPLICIT_CONTEXT
  PERL_MALLOC_WRAP
  PERL_OP_PARENT
  PERL_PRESERVE_IVUV
  PERL_USE_DEVEL
  PERL_USE_SAFE_PUTENV
  USE_64_BIT_ALL
  USE_64_BIT_INT
  USE_ITHREADS
  USE_LARGE_FILES
  USE_LOCALE
  USE_LOCALE_COLLATE
  USE_LOCALE_CTYPE
  USE_LOCALE_NUMERIC
  USE_LOCALE_TIME
  USE_PERLIO
  USE_PERL_ATOF
  USE_REENTRANT_API
  Locally applied patches​:
  uncommitted-changes
  Built under cygwin
  Compiled at Jul 16 2016 17​:06​:23
  @​INC​:
  lib
  /usr/local/lib/perl5/site_perl/5.25.3/cygwin-thread-multi
  /usr/local/lib/perl5/site_perl/5.25.3
  /usr/local/lib/perl5/5.25.3/cygwin-thread-multi
  /usr/local/lib/perl5/5.25.3
  .