Precompiled headers: supply "-x c-header" or "-x c++-header" if available.

Some older versions of gcc give "all.hpp: linker input file unused
because linking not done" otherwise, because they don't realize files
named *.hpp are headers.

Reported-by: Wayne Scott <wsc9tt@gmail.com>

redoconf/_compile.od

@@ -6,9 +6,10 @@ cat >$3 <<-EOF
 	t="\$1" d="\$2" i="\$3"
 	IFS="$NL"
 	set -f
-	\$CC -o "\$t" -c "\$i" \\
+	\$CC -o "\$t" \\
 	  -MMD -MF "\$d" \\
-	  \$CPPFLAGS \$CFLAGS \$CXXFLAGS \$FLAGS_PCH \$xCFLAGS \$OPTFLAGS
+	  \$CPPFLAGS \$CFLAGS \$CXXFLAGS \$FLAGS_PCH \$xCFLAGS \$OPTFLAGS \\
+	  -c "\$i"
 EOF
 chmod a+x "$3"
 redo-stamp <$3

redoconf/compile.od

@@ -31,6 +31,7 @@ cat >$3 <<-EOF
 	    CXXFLAGS=
 	    PCH1=$(shquote "$CFLAGS_PCH")
 	    PCH2=$(shquote "$CFLAGS_PCH_FPIC")
+	    PCH3=$(shquote "$CFLAGS_PCH_LANG")
 	    ;;
 	  *)
 	    CC=$(shquote "$CXX")
@@ -39,11 +40,13 @@ cat >$3 <<-EOF
 	    CXXFLAGS=$(shquote "$CXXFLAGS")
 	    PCH1=$(shquote "$CXXFLAGS_PCH")
 	    PCH2=$(shquote "$CXXFLAGS_PCH_FPIC")
+	    PCH3=$(shquote "$CXXFLAGS_PCH_LANG")
 	    ;;
 	esac
 	case \$PCH in
 	  1) FLAGS_PCH=\$PCH1 ;;
 	  2) FLAGS_PCH=\$PCH2 ;;
+	  3) FLAGS_PCH=\$PCH3 ;;
 	esac
 	. ./_compile
 EOF

redoconf/default.do.sh

@@ -116,14 +116,14 @@ case $1 in
 	;;
     *.h.fpic.gch|*.hpp.fpic.gch)
 	src="$S/${1%.fpic.gch}"
-	xCFLAGS="-fPIC" PCH="" dep="" compile "$@"
+	xCFLAGS="-fPIC" PCH="3" dep="" compile "$@"
 	# precompiled header is "unchanged" if its component
 	# headers are unchanged.
 	cat ${deps#*:} | tee $1.stamp | redo-stamp
 	;;
     *.h.gch|*.hpp.gch)
 	src="$S/${1%.gch}"
-	xCFLAGS="" PCH="" dep="" compile "$@"
+	xCFLAGS="" PCH="3" dep="" compile "$@"
 	# precompiled header is "unchanged" if its component
 	# headers are unchanged.
 	cat ${deps#*:} | tee $1.stamp | redo-stamp

redoconf/rc/default.h.precompiled.rc.od

@@ -4,13 +4,20 @@ rc_include rc/CC.rc
 base="${1#rc/}"
 src="${base%.h.precompiled.rc}"
 
+# Check whether the compiler supports forcing
+# the language type.  Some versions of gcc
+# need this in order to precompile headers
+# named *.hpp.
+x="-x${NL}c-header"
+if (appendln CFLAGS "$x" && rc_compile cc nolink); then
+	rc_appendln CFLAGS_PCH_LANG "$x"
+fi
+
 # The existence of the specific gcc warning about
 # precompiled headers is a pretty good indicator
 # that they are supported in the way we expect.
 rc_appendln CFLAGS "-Winvalid-pch"
 if rc_compile cc nolink; then
-    rc_appendln CFLAGS "$x"
-
     rc_appendln CFLAGS_PCH "-include$NL$src.h"
     rc_appendln CFLAGS_PCH_FPIC "-include$NL$src.h.fpic"
 

redoconf/rc/default.hpp.precompiled.rc.od

@@ -4,13 +4,20 @@ rc_include rc/CXX.rc
 base="${1#rc/}"
 src="${base%.hpp.precompiled.rc}"
 
+# Check whether the compiler supports forcing
+# the language type.  Some versions of gcc
+# need this in order to precompile headers
+# named *.hpp.
+x="-x${NL}c++-header"
+if (appendln CXXFLAGS "$x" && rc_compile cxx nolink); then
+	rc_appendln CXXFLAGS_PCH_LANG "$x"
+fi
+
 # The existence of the specific gcc warning about
 # precompiled headers is a pretty good indicator
 # that they are supported in the way we expect.
-appendln CXXFLAGS "-Winvalid-pch"
+rc_appendln CXXFLAGS "-Winvalid-pch"
 if rc_compile cxx nolink; then
-    rc_appendln CXXFLAGS "$x"
-
     rc_appendln CXXFLAGS_PCH "-include$NL$src.hpp"
     rc_appendln CXXFLAGS_PCH_FPIC "-include$NL$src.hpp.fpic"