Solaris and "implicit declaration of function ‘isinf’"

Description
I'm building Perl 5.30.1 from the release tarball on Solaris 11.3 i86pc.

The build is producing several warnings like:

gcc -c -DPERL_CORE -g2 -O2 -m64 -march=native -fPIC -pthread -fwrapv -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -I/usr/gnu/include -m64 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -DPERL_USE_SAFE_PUTENV -O -Wall -Werror=declaration-after-statement -Werror=pointer-arith -Wextra -Wc++-compat -Wwrite-strings -fPIC numeric.c
numeric.c: In function ‘S_my_atof_infnan’:
numeric.c:1461:21: warning: implicit declaration of function ‘isinf’ [-Wimplicit-function-declaration]
                     if (Perl_isinf(nv))
                     ^

According to the Sun man pages:

$ man isinf
...

NAME
       isinf - test for infinity

SYNOPSIS
       c99 [ flag... ] file... -lm [ library... ]
       #include <math.h>

       int isinf(real-floating x);

DESCRIPTION
       The  isinf() macro determines whether its argument value is an infinity
       (positive or negative). First, an  argument  represented  in  a  format
       wider  than  its  semantic type is converted to its semantic type. Then
       determination is based on the type of the argument.

Steps to Reproduce
Build on Solaris 11.3.

I perform 64-bit builds on Solaris according to 64-bit Computing | When to Use 64-bit in the Developers Guide. For Autotools projects I use amd64-sun-solaris. I also add -m64 to CFLAGS, CXXFLAGS and LDFLAGS.

Be careful of the "converted to its semantic type..." in the man page. I think the three choices for real-floating are long int, float and double. I've seen missing math prototypes on Solaris because the compiler did not promote as expected and a simple cast was missing.

Expected behavior
A clean compile

Perl configuration

$ PERL5LIB=./lib ./perl -V
Summary of my perl5 (revision 5 version 30 subversion 1) configuration:
   
  Platform:
    osname=solaris
    osvers=2.11
    archname=i86pc-solaris-64
    uname='sunos solaris 5.11 11.3 i86pc i386 i86pc solaris '
    config_args='-des -Dprefix=/usr/local -Dlibdir=/usr/local/lib -Dpkgconfig=/usr/local/lib/pkgconfig -Dcc=gcc -Dcxx=g++ -Acppflags=-I/usr/local/include -DNDEBUG -Accflags=-g2 -O2 -m64 -march=native -fPIC -pthread -Acxxflags=-g2 -O2 -m64 -march=native -fPIC -pthread -Aldflags=-L/usr/local/lib -m64 -Wl,-R,'XXORIGIN/../lib' -Wl,-R,/usr/local/lib -Dextras=Text::Template Test::More'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=undef
    usemultiplicity=undef
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='gcc'
    ccflags ='-g2 -O2 -m64 -march=native -fPIC -pthread -fwrapv -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -I/usr/gnu/include -m64 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -DPERL_USE_SAFE_PUTENV'
    optimize='-O'
    cppflags='-I/usr/local/include -DNDEBUG -g2 -O2 -m64 -march=native -fPIC -pthread -fwrapv -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -I/usr/gnu/include'
    ccversion=''
    gccversion='4.8.2'
    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='gcc'
    ldflags =' -L/usr/local/lib -m64 -Wl,-R,XXORIGIN/../lib -Wl,-R,/usr/local/lib -fstack-protector -L/usr/gnu/lib -m64 '
    libpth=/usr/local/lib /usr/gcc/4.8/lib /usr/gcc/4.8/lib/gcc/i386-pc-solaris2.11/4.8.2/include-fixed /usr/lib /usr/gnu/lib /usr/ccs/lib
    libs=-lpthread -lsocket -lnsl -lgdbm -ldb -ldl -lm -lc
    perllibs=-lpthread -lsocket -lnsl -ldl -lm -lc
    libc=/lib/libc.so
    so=so
    useshrplib=true
    libperl=libperl.so
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='  -R /usr/local/lib/perl5/5.30.1/i86pc-solaris-64/CORE'
    cccdlflags='-fPIC'
    lddlflags=' -shared -m64 -L/usr/local/lib -Wl,-R,XXORIGIN/../lib -Wl,-R,/usr/local/lib -L/usr/gnu/lib -fstack-protector'

Characteristics of this binary (from libperl): 
  Compile-time options:
    HAS_TIMES
    PERLIO_LAYERS
    PERL_COPY_ON_WRITE
    PERL_DONT_CREATE_GVSV
    PERL_MALLOC_WRAP
    PERL_OP_PARENT
    PERL_PRESERVE_IVUV
    PERL_USE_SAFE_PUTENV
    USE_64_BIT_ALL
    USE_64_BIT_INT
    USE_LARGE_FILES
    USE_LOCALE
    USE_LOCALE_COLLATE
    USE_LOCALE_CTYPE
    USE_LOCALE_NUMERIC
    USE_LOCALE_TIME
    USE_PERLIO
    USE_PERL_ATOF
  Built under solaris
  Compiled at Feb  6 2020 15:14:41
  %ENV:
    PERL5LIB="./lib"
  @INC:
    ./lib
    /usr/local/lib/perl5/site_perl/5.30.1/i86pc-solaris-64
    /usr/local/lib/perl5/site_perl/5.30.1
    /usr/local/lib/perl5/5.30.1/i86pc-solaris-64
    /usr/local/lib/perl5/5.30.1

@jkeenan - I think this is the last of the trivial reports for lesser used platforms. I've got to move on to other tasks.

Start in numeric.c at S_my_atof_infnan and reproduce the Perl 5.30.1 64-bit build on Solaris 11.3 using the reported GCC configuration. Trace how Perl_isinf and the Solaris isinf declaration are detected or included; done means the Solaris build completes without the implicit-declaration warning.