guix: Check mingw symbols, improve SSP fix docs

contrib/guix/libexec/build.sh

@@ -211,7 +211,7 @@ export PATH="${BASEPREFIX}/${HOST}/native/bin:${PATH}"
     make -C src --jobs=1 check-security ${V:+V=1}
 
     case "$HOST" in
-        *linux*)
+        *linux*|*mingw*)
             # Check that executables only contain allowed gcc, glibc and libstdc++
             # version symbols for Linux distro back-compatibility.
             make -C src --jobs=1 check-symbols  ${V:+V=1}

contrib/guix/manifest.scm

@@ -26,7 +26,10 @@
 
 (define (make-ssp-fixed-gcc xgcc)
   "Given a XGCC package, return a modified package that uses the SSP function
-from glibc instead of from libssp.so. Taken from:
+from glibc instead of from libssp.so. Our `symbol-check' script will complain if
+we link against libssp.so, and thus will ensure that this works properly.
+
+Taken from:
 http://www.linuxfromscratch.org/hlfs/view/development/chapter05/gcc-pass1.html"
   (package
    (inherit xgcc)