configure: Make C++ test work with --enable-werror

gcc's C++ compiler complains about being passed some -W options
which make sense for C but not for C++. This means we mustn't try
a C++ compile with QEMU_CFLAGS, but only with a filtered version
that removes the offending options. This filtering was already being
done for uses of C++ in the build itself, but was omitted for the
"does C++ work?" configure test. This only showed up when doing
builds which explicitly enabled -Werror with --enable-werror,
because the "do the compilers work" tests were mistakenly placed
above the "default werror based on whether compiling from git" code.
Another error in this category is that clang warns if you ask it to
compile C++ code from a file named "foo.c". Further, because we
were running do_cc in a subshell in the condition part of an "if",
the error_exit inside do_compiler wouldn't terminate configure and
we would plunge on regardless. Fix this complex of errors:

1. Move the default-werror code up so that there are no invocations
of compile_object and friends between it and the point where we
set $werror explicitly based on the --enable-werror command line
option.

2. Provide a mechanism for filtering QEMU_CFLAGS to create
QEMU_CXXFLAGS, and use it for the test we run here.

3. Provide a do_cxx function to run a test with the C++ compiler
rather than doing cute tricks with subshells and do_cc.

4. Use a new temporary file TMPCXX for the C++ program fragment.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 1393352869-22257-1-git-send-email-peter.maydell@linaro.org
Tested-by: NAndreas Färber <afaerber@suse.de>

configure

@@ -14,13 +14,14 @@ fi
 TMPC="${TMPDIR1}/qemu-conf-${RANDOM}-$$-${RANDOM}.c"
 TMPB="qemu-conf-${RANDOM}-$$-${RANDOM}"
 TMPO="${TMPDIR1}/${TMPB}.o"
+TMPCXX="${TMPDIR1}/${TMPB}.cxx"
 TMPL="${TMPDIR1}/${TMPB}.lo"
 TMPA="${TMPDIR1}/lib${TMPB}.la"
 TMPE="${TMPDIR1}/qemu-conf-${RANDOM}-$$-${RANDOM}.exe"
 
 # NB: do not call "exit" in the trap handler; this is buggy with some shells;
 # see <1285349658-3122-1-git-send-email-loic.minier@linaro.org>
-trap "rm -f $TMPC $TMPO $TMPE" EXIT INT QUIT TERM
+trap "rm -f $TMPC $TMPO $TMPCXX $TMPE" EXIT INT QUIT TERM
 rm -f config.log
 
 # Print a helpful header at the top of config.log
@@ -54,10 +55,13 @@ error_exit() {
     exit 1
 }
 
-do_cc() {
-    # Run the compiler, capturing its output to the log.
-    echo $cc "$@" >> config.log
-    $cc "$@" >> config.log 2>&1 || return $?
+do_compiler() {
+    # Run the compiler, capturing its output to the log. First argument
+    # is compiler binary to execute.
+    local compiler="$1"
+    shift
+    echo $compiler "$@" >> config.log
+    $compiler "$@" >> config.log 2>&1 || return $?
     # Test passed. If this is an --enable-werror build, rerun
     # the test with -Werror and bail out if it fails. This
     # makes warning-generating-errors in configure test code
@@ -71,14 +75,39 @@ do_cc() {
            return 0
         ;;
     esac
-    echo $cc -Werror "$@" >> config.log
-    $cc -Werror "$@" >> config.log 2>&1 && return $?
+    echo $compiler -Werror "$@" >> config.log
+    $compiler -Werror "$@" >> config.log 2>&1 && return $?
     error_exit "configure test passed without -Werror but failed with -Werror." \
         "This is probably a bug in the configure script. The failing command" \
         "will be at the bottom of config.log." \
         "You can run configure with --disable-werror to bypass this check."
 }
 
+do_cc() {
+    do_compiler "$cc" "$@"
+}
+
+do_cxx() {
+    do_compiler "$cxx" "$@"
+}
+
+update_cxxflags() {
+    # Set QEMU_CXXFLAGS from QEMU_CFLAGS by filtering out those
+    # options which some versions of GCC's C++ compiler complain about
+    # because they only make sense for C programs.
+    QEMU_CXXFLAGS=
+    for arg in $QEMU_CFLAGS; do
+        case $arg in
+            -Wstrict-prototypes|-Wmissing-prototypes|-Wnested-externs|\
+            -Wold-style-declaration|-Wold-style-definition|-Wredundant-decls)
+                ;;
+            *)
+                QEMU_CXXFLAGS=${QEMU_CXXFLAGS:+$QEMU_CXXFLAGS }$arg
+                ;;
+        esac
+    done
+}
+
 compile_object() {
   do_cc $QEMU_CFLAGS -c -o $TMPO $TMPC
 }
@@ -1335,6 +1364,19 @@ if test "$ARCH" = "unknown"; then
     fi
 fi
 
+# Consult white-list to determine whether to enable werror
+# by default.  Only enable by default for git builds
+z_version=`cut -f3 -d. $source_path/VERSION`
+
+if test -z "$werror" ; then
+    if test -d "$source_path/.git" -a \
+        "$linux" = "yes" ; then
+        werror="yes"
+    else
+        werror="no"
+    fi
+fi
+
 # check that the C compiler works.
 cat > $TMPC <<EOF
 int main(void) { return 0; }
@@ -1355,14 +1397,16 @@ EOF
 
     compile_object
 
-    cat > $TMPC <<EOF
+    cat > $TMPCXX <<EOF
 extern "C" {
    int c_function(void);
 }
 int c_function(void) { return 42; }
 EOF
 
-    if (cc=$cxx do_cc $QEMU_CFLAGS -o $TMPE $TMPC $TMPO $LDFLAGS); then
+    update_cxxflags
+
+    if do_cxx $QEMU_CXXFLAGS -o $TMPE $TMPCXX $TMPO $LDFLAGS; then
         # C++ compiler $cxx works ok with C compiler $cc
         :
     else
@@ -1375,19 +1419,6 @@ else
     cxx=
 fi
 
-# Consult white-list to determine whether to enable werror
-# by default.  Only enable by default for git builds
-z_version=`cut -f3 -d. $source_path/VERSION`
-
-if test -z "$werror" ; then
-    if test -d "$source_path/.git" -a \
-        "$linux" = "yes" ; then
-        werror="yes"
-    else
-        werror="no"
-    fi
-fi
-
 gcc_flags="-Wold-style-declaration -Wold-style-definition -Wtype-limits"
 gcc_flags="-Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers $gcc_flags"
 gcc_flags="-Wmissing-include-dirs -Wempty-body -Wnested-externs $gcc_flags"