Does not build with GCC 7.3.0 on Cygwin due to -Werror=sign-compare

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Problem Description

Trying to build OpenSC on Cygwin fails.

Proposed Resolution

Write warning-free code.

Steps to reproduce

1. git clone
2. patch configure.ac (possibly a better solution to Cygwin/ssh with opensc-pkcs11.dll: Segmentation fault #607)

diff --git a/configure.ac b/configure.ac
index 3769757a..7fba83eb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -104,7 +104,7 @@ case "${host}" in
                        WIN32="yes"
                else
                        AC_MSG_RESULT([Using cygwin])
-                       CPPFLAGS="${CPPFLAGS} -DCRYPTOKI_FORCE_WIN32"
+                       # CPPFLAGS="${CPPFLAGS} -DCRYPTOKI_FORCE_WIN32"
                        WIN_LIBPREFIX="cyg"
                        AC_DEFINE([USE_CYGWIN], [1], [Define if you are on Cygwin])
                fi

3. ./bootstrap
4. mkdir __build; cd __build; ../configure
5. make -j4 -k
   ...

make[3]: Entering directory '/home/Csaba/workspace/OpenSC/__build/src/libopensc'
  CC       libopensc_la-reader-pcsc.lo
../../../src/libopensc/reader-pcsc.c: In function ‘pcsc_cancel’:
../../../src/libopensc/reader-pcsc.c:719:27: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
  if (gpriv->pcsc_wait_ctx != -1) {
                           ^~
../../../src/libopensc/reader-pcsc.c: In function ‘pcsc_wait_for_event’:
../../../src/libopensc/reader-pcsc.c:1501:27: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
  if (gpriv->pcsc_wait_ctx == -1) {
                           ^~
cc1: all warnings being treated as errors
make[3]: *** [Makefile:1313: libopensc_la-reader-pcsc.lo] Error 1
make[3]: Target 'all' not remade because of errors.
make[3]: Leaving directory '/home/Csaba/workspace/OpenSC/__build/src/libopensc'
Making all in pkcs11
make[3]: Entering directory '/home/Csaba/workspace/OpenSC/__build/src/pkcs11'
make[3]: *** No rule to make target '../../src/libopensc/libopensc.la', needed by 'opensc-pkcs11.la'.
make[3]: Target 'all' not remade because of errors.
make[3]: Leaving directory '/home/Csaba/workspace/OpenSC/__build/src/pkcs11'
Making all in tools
make[3]: Entering directory '/home/Csaba/workspace/OpenSC/__build/src/tools'
  CCLD     opensc-tool.exe
libtool:   error: cannot find the library '../../src/libopensc/libopensc.la' or unhandled argument '../../src/libopensc/libopensc.la'
make[3]: *** [Makefile:855: opensc-tool.exe] Error 1
  CCLD     opensc-explorer.exe
libtool:   error: cannot find the library '../../src/libopensc/libopensc.la' or unhandled argument '../../src/libopensc/libopensc.la'

The fact that it tried to make all in tools after building libopensc failed suggests a problem in the build system itself (libopensc should be a prerequisite of pkcs and tools).

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/lto-wrapper.exe
Target: x86_64-pc-cygwin
Configured with: /cygdrive/i/szsz/tmpp/gcc/gcc-7.3.0-3.x86_64/src/gcc-7.3.0/configure --srcdir=/cygdrive/i/szsz/tmpp/gcc/gcc-7.3.0-3.x86_64/src/gcc-7.3.0 --prefix=/usr --exec-prefix=/usr --localstatedir=/var --sysconfdir=/etc --docdir=/usr/share/doc/gcc --htmldir=/usr/share/doc/gcc/html -C --build=x86_64-pc-cygwin --host=x86_64-pc-cygwin --target=x86_64-pc-cygwin --without-libiconv-prefix --without-libintl-prefix --libexecdir=/usr/lib --enable-shared --enable-shared-libgcc --enable-static --enable-version-specific-runtime-libs --enable-bootstrap --enable-__cxa_atexit --with-dwarf2 --with-tune=generic --enable-languages=ada,c,c++,fortran,lto,objc,obj-c++ --enable-graphite --enable-threads=posix --enable-libatomic --enable-libcilkrts --enable-libgomp --enable-libitm --enable-libquadmath --enable-libquadmath-support --disable-libssp --enable-libada --disable-symvers --with-gnu-ld --with-gnu-as --with-cloog-include=/usr/include/cloog-isl --without-libiconv-prefix --without-libintl-prefix --with-system-zlib --enable-linker-build-id --with-default-libstdcxx-abi=gcc4-compatible --enable-libstdcxx-filesystem-ts
Thread model: posix
gcc version 7.3.0 (GCC)

[frankmorgner]

Feel free to make a pull request