configure: detect warning opts missing in older compilers.

* configure (diag_flags): Remove -Wvla and
-Werror=declaration-after-statement.
(diag_flags_given): New variable.
New test: if diag_flags_given indicates that diag_flags were
not specified by the user, then we try to add additional
flags, subject to them being available, which we test by
compiling the hello-world program with those flags. We rely on
the hello-world program being left over by the previous
compiler sanity check.

1 files changed, 16 insertions, 2 deletions

diff --git a/configure b/configure
index 561976e6..68ceb513 100755
--- a/configure
+++ b/configure
@@ -167,8 +167,9 @@ nm='$(cross)$(tool_prefix)nm'
 opt_flags='-O2 -fno-stack-protector'
 lang_flags='-D_XOPEN_SOURCE=700 -D_POSIX_C_SOURCE=200112 -D_GNU_SOURCE'
 diag_flags="-Wall -Wextra -Werror=implicit-function-declaration \
-           -Werror=missing-prototypes -Werror=strict-prototypes -Werror=vla \
-           -Werror=old-style-definition -Werror=declaration-after-statement"
+           -Werror=missing-prototypes -Werror=strict-prototypes \
+           -Werror=old-style-definition"
+diag_flags_given=
 debug_flags=-g
 debug_only_flags=-DTXR_DEBUG
 debug_also=
@@ -1055,6 +1056,19 @@ else
   printf "okay\n"
 fi
 
+if ! [ $diag_flags_given ] ; then
+  for opt in -Werror=vla -Werror=declaration-after-statement ; do
+    printf "Checking for %s ... " $opt
+
+    if conftest EXTRA_FLAGS="-Werror $opt" ; then
+      printf "yes\n"
+      diag_flags="$diag_flags $opt"
+    else
+      printf "no\n"
+    fi
+  done
+fi
+
 printf "Checking compiler version for various workarounds ... "
 
 output=$($make conftest.ccver)