build: avoid clang's colored diagnostics in the test suite

The syncline tests, which try to recognize compiler diagnostics,
are confused by escapes for colors.

* configure.ac (warn_tests): New, to factor the warnings for both
C and C++ tests.
Add -fno-color-diagnostics to it.
* tests/local.at (AT_TEST_TABLES_AND_PARSE): Do not remove glue
together compiler flags.

configure.ac

@@ -86,6 +86,12 @@ if test "$enable_gcc_warnings" = yes; then
     -Wformat -Wpointer-arith -Wwrite-strings'
   warn_c='-Wbad-function-cast -Wshadow -Wstrict-prototypes'
   warn_cxx='-Wnoexcept'
+  # Warnings for the test suite only.
+  #
+  # -fno-color-diagnostics: Clang's use of colors in the error
+  # messages is confusing the tests looking at the compiler's output
+  # (e.g., synclines.at).
+  warn_tests='-Wundef -pedantic -fno-color-diagnostics'
 
   AC_LANG_PUSH([C])
   # Clang supports many of GCC's -W options, but only issues warnings
@@ -114,12 +120,13 @@ if test "$enable_gcc_warnings" = yes; then
     AS_VAR_APPEND([WARN_CFLAGS], [" $WARN_CFLAGS_TEST"])
 
   # Warnings for the test suite only.
-  gl_WARN_ADD([-Wundef], [WARN_CFLAGS_TEST])
-  gl_WARN_ADD([-pedantic], [WARN_CFLAGS_TEST])
+  for i in $warn_tests;
+  do
+    gl_WARN_ADD([$i], [WARN_CFLAGS_TEST])
+  done
   CFLAGS=$save_CFLAGS
   AC_LANG_POP([C])
 
-
   AC_LANG_PUSH([C++])
   save_CXXFLAGS=$CXXFLAGS
   gl_WARN_ADD([-Werror=unknown-warning-option], [CXXFLAGS])
@@ -131,8 +138,10 @@ if test "$enable_gcc_warnings" = yes; then
               [AC_LANG_PROGRAM([], [nullptr])])
   gl_WARN_ADD([-Werror], [WERROR_CXXFLAGS])
   # Warnings for the test suite only.
-  gl_WARN_ADD([-Wundef], [WARN_CXXFLAGS_TEST])
-  gl_WARN_ADD([-pedantic], [WARN_CXXFLAGS_TEST])
+  for i in $warn_tests;
+  do
+    gl_WARN_ADD([$i], [WARN_CXXFLAGS_TEST])
+  done
   # Clang++ 3.2+ reject C code generated by Flex.
   gl_WARN_ADD([-Wno-null-conversion], [WARN_NO_NULL_CONVERSION_CXXFLAGS])
   # Variants break strict aliasing analysis.