CI: use ICC again

See https://github.com/nemequ/icc-travis/issues/15.
Thanks to Jeff Hammond and Evan Nemerson for their help.

* configure.ac (warn_common): Disable dubious warnings.
* .travis.yml: Use ICC again.

configure.ac

@@ -102,13 +102,41 @@ if test "$enable_gcc_warnings" = yes; then
   # -Wno-tautological-constant-out-of-range-compare for Clang 3.3 and
   # 3.4 on GNU/Linux that choke on intprops.h's INT_MULTIPLY_WRAPV,
   # etc.
+  #
+  # ICC: -wr188
+  #
+  # 1669  warnings warnings_default =
+  # ../src/complain.c(318): error #188: enumerated type mixed with another type
+  # 1670      Wconflicts_sr | Wconflicts_rr | Wdeprecated | Wother;
+  # 1671      ^
+  # 1672
+  # 1673../src/complain.c(393): error #188: enumerated type mixed with another type
+  # 1674          warnings w = 1 << wbit;
+  # 1675                       ^
+  #
+  # ICC: -wr3179
+  #
+  # char const *usefulness
+  #   = rule_useless_in_grammar_p (&rules[r]) ? "useless-in-grammar"
+  #   : rule_useless_in_parser_p (&rules[r])  ? "useless-in-parser"
+  #   :                                         "useful";
+  #
+  # gives
+  #
+  # error #3179: deprecated conversion of string literal to char* (should be const char*)
+  #
+  # ICC: -wr2259 (that's in C, in spite of what the error messages which seems to be about C++).
+  # error #2259: non-pointer conversion from "int" to "yybool={signed char}" may lose significant bits
+  #   yybool yynormal YY_ATTRIBUTE_UNUSED = yystackp->yysplitPoint == YY_NULLPTR;
+  #                                         ^
   warn_common='-Wall -Wextra
     -Wcast-align -Wchar-subscripts
     -fparse-all-comments -Wdocumentation
     -Wformat -Wimplicit-fallthrough -Wnull-dereference
     -Wno-sign-compare -Wno-tautological-constant-out-of-range-compare
     -Wpointer-arith -Wshadow
-    -Wwrite-strings'
+    -Wwrite-strings
+    -wr188 -wr2259 -wr3179'
   warn_c='-Wbad-function-cast -Wstrict-prototypes'
   warn_cxx='-Wextra-semi -Wnoexcept -Wold-style-cast -Wundefined-func-template
     -Wweak-vtables'