Configure fails to find libquadmath on cygwin

[tonycoz]

Module:

Description

Configure fails to find libquadmath on cygwin:

$ ./Configure -des -Dusedevel -Dusequadmath
...
*** You requested the use of the quadmath library, but you
*** do not seem to have the quadmath library installed.
*** Cannot continue, aborting.

tony@phobos ~/dev/perl/git/perl
$ git describe
v5.31.11-21-g6319bad96e

tony@phobos ~/dev/perl/git/perl
$ uname -a
CYGWIN_NT-10.0 phobos 3.1.4(0.340/5/3) 2020-02-19 08:49 x86_64 Cygwin

But the compiler has no problem finding it:

$ cc -otestquad.exe testquad.c -lquadmath

$ ./testquad
2.000000

$ cat testquad.c
#include <quadmath.h>
#include <stdio.h>

int main() {
  __float128 q = ceilq(1.2);
  printf("%f\n", (double)q);
  return 0;
}

I considered applying a change to the hints file as is done in hints/linux.sh that asks gcc for -print-search-dirs, but to find quadmath this requires adding gcc's version specific directories and means that if a new gcc is installed we can end up linking an extension against different versions of libraries than the executable or libperl shared library.

But the main problem I see is we have a tool that can find libraries (the compiler/linker) and in general it's smarter about it than Configure is - since it will only accept libraries that will match the current build, while Configure only checks names.

Perl configuration
From a build without -Dusequadmath:

Summary of my perl5 (revision 5 version 31 subversion 12) configuration:
  Commit id: 6319bad96eba493c5f3bc7857e1c58687c461d70
  Platform:
    osname=cygwin
    osvers=3.1.4(0.34053)
    archname=cygwin-thread-multi
    uname='cygwin_nt-10.0 phobos 3.1.4(0.34053) 2020-02-19 08:49 x86_64 cygwin '
    config_args='-des -Dusedevel'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='gcc'
    ccflags ='-DPERL_USE_SAFE_PUTENV -U__STRICT_ANSI__ -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -D_FORTIFY_SOURCE=2'
    optimize='-O3'
    cppflags='-DPERL_USE_SAFE_PUTENV -U__STRICT_ANSI__ -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong'
    ccversion=''
    gccversion='9.3.0'
    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/9.3.0/../../../../lib/../include/w32api /usr/local/lib /lib
    libs=-lpthread -ldl
    perllibs=-lpthread -ldl
    libc=/usr/lib/libc.a
    so=dll
    useshrplib=true
    libperl=cygperl5_31_12.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_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
    USE_THREAD_SAFE_LOCALE
  Built under cygwin
  Compiled at May 12 2020 14:34:44
  @INC:
    lib
    /usr/local/lib/perl5/site_perl/5.31.12/cygwin-thread-multi
    /usr/local/lib/perl5/site_perl/5.31.12
    /usr/local/lib/perl5/5.31.12/cygwin-thread-multi
    /usr/local/lib/perl5/5.31.12