After upgrading to g++ (GCC) 6.4.0, I am being forced to
reload/rebuild a bunch of Perl modules that had been working fine.  I
had the same issue with GCC 6.3.  One module is particularly
troublesome.  I have the openssl library installed.  This is with the
64-bit Cygwin.  I attached cygcheck.out

I would roll back to GCC 5.x but that doesn't appear to be an option in Setup.

# cpan install Net::SSLeay

Running Mkbootstrap for Net::SSLeay ()
chmod 644 "SSLeay.bs"
"/usr/bin/perl.exe" "-Iinc" "/usr/lib/perl5/5.24/ExtUtils/xsubpp"
-typemap "/usr/lib/perl5/5.24/ExtUtils/typemap" -typemap "typemap"
SSLeay.xs > SSLeay.xsc && mv SSLeay.xsc SSLeay.c
gcc -c   -DPERL_USE_SAFE_PUTENV -D_GNU_SOURCE -U__STRICT_ANSI__ -ggdb
-O2 -pipe -Wimplicit-function-declaration
-fdebug-prefix-map=/mnt/share/maint/perl.x86_64/build=/usr/src/debug/perl-5.24.1-1
-fdebug-prefix-map=/mnt/share/maint/perl.x86_64/src/perl-5.24.1=/usr/src/debug/perl-5.24.1-1
-fwrapv -fno-strict-aliasing -fstack-protector-strong
-D_FORTIFY_SOURCE=2 -DUSEIMPORTLIB -O3   -DVERSION=\"1.81\"
-DXS_VERSION=\"1.81\"
"-I/usr/lib/perl5/5.24/x86_64-cygwin-threads/CORE"   SSLeay.c
SSLeay.xs:163:25: fatal error: openssl/err.h: No such file or directory
 #include <openssl/err.h>
                         ^
compilation terminated.
make: *** [Makefile:357: SSLeay.o] Error 1
  MIKEM/Net-SSLeay-1.81.tar.gz
  /usr/bin/make -- NOT OK

-- 
Jim Reisert AD1C, <jjreisert@alum.mit.edu>, http://www.ad1c.us