configure: quit on trying to get ICC and Flex be friends

The CI is using Flex 2.5.35.  And ICC is too picky for it.  Let's stop
making these warnings errors.  I wish I could disable them in the
source files using the ICC version and the Flex version, but ICC's
pragma support is unclear, and I'm tired of fighting it.

* configure.ac (FLEX_SCANNER_CXXFLAGS): Make warnings warnings.
* examples/c++/local.mk: Comment changes.

configure.ac

@@ -162,15 +162,11 @@ if test "$enable_gcc_warnings" = yes; then
   do
     gl_WARN_ADD([$i], [WARN_CXXFLAGS_TEST])
   done
-  # Clang++ 3.2+ rejects C code generated by Flex.
-  gl_WARN_ADD([-Wno-null-conversion], [FLEX_SCANNER_CXXFLAGS])
-  # So does G++ 4.8...
-  gl_WARN_ADD([-Wno-sign-compare], [FLEX_SCANNER_CXXFLAGS])
-  # ... possibly in std=c++11 mode.
-  gl_WARN_ADD([-Wno-zero-as-null-pointer-constant], [FLEX_SCANNER_CXXFLAGS])
-  CXXFLAGS=$save_CXXFLAGS
+  # Too many compilers complain about Flex generated code.
+  gl_WARN_ADD([-Wno-error], [FLEX_SCANNER_CXXFLAGS])
   # Clang++ deprecates compiling C.
   gl_WARN_ADD([-Wno-deprecated], [WNO_DEPRECATED_CXXFLAGS])
+  CXXFLAGS=$save_CXXFLAGS
   AC_LANG_POP([C++])
 fi
 

examples/c++/local.mk

@@ -46,6 +46,7 @@ endif
 
 check_PROGRAMS += %D%/variant
 nodist_%C%_variant_SOURCES = %D%/variant.yy
+# Don't use gnulib's system headers.
 %C%_variant_CPPFLAGS = -I$(top_builddir)
 dist_TESTS += %D%/variant.test
 %D%/variant.cc: $(BISON_IN) $(dist_pkgdata_DATA)
@@ -54,6 +55,7 @@ if ENABLE_CXX11
   check_PROGRAMS += %D%/variant-11
   nodist_%C%_variant_11_SOURCES = %D%/variant-11.yy
   %C%_variant_11_CXXFLAGS = $(CXX11_CXXFLAGS)
+  # Don't use gnulib's system headers.
   %C%_variant_11_CPPFLAGS = -I$(top_builddir)
   dist_TESTS += %D%/variant-11.test
   %D%/variant-11.cc: $(BISON_IN) $(dist_pkgdata_DATA)