Revert "use -isystem instead of -I for 3rd party headers"

This reverts commit 151abb8b.

Unfortunately has the side effect that it will prevent GCC from
generating makefile dependencies for headers from bundled external
libraries, which breaks incremental builds horribly.
(Retain the uses in configure for real system headers).

Conflicts:
	RepositoryExternal.mk
	configure.ac

Change-Id: I149db1d402fa18bdc470f90dee846cfb5158237e

config_host.mk.in

@@ -251,7 +251,6 @@ export INPATH_FOR_BUILD=@INPATH_FOR_BUILD@
 export INSTALLDIR=@INSTALLDIR@
 export INSTALLDIRNAME=@INSTALLDIRNAME@
 export INTRO_BITMAP=@INTRO_BITMAP@
-export ISYSTEM=@ISYSTEM@
 export JAVACISGCJ=@JAVACISGCJ@
 export JAVACISKAFFE=@JAVACISKAFFE@
 export JAVACOMPILER=@JAVACOMPILER@

configure.ac

@@ -2883,6 +2883,7 @@ fi
 AC_SUBST(HAVE_LD_BSYMBOLIC_FUNCTIONS)
 
 # Use -isystem (gcc) if possible, to avoid warnigs in 3rd party headers.
+# NOTE: must _not_ be used for bundled external libraries!
 ISYSTEM=
 if test "$GCC" = "yes"; then
     AC_MSG_CHECKING( for -isystem )
@@ -3973,7 +3974,7 @@ else
     WORKDIR=${BUILDDIR}/workdir/${INPATH}
 fi
 OUTDIR="${SOLARVER}/${INPATH}"
-SOLARINC="-I. ${ISYSTEM}${SOLARVER}/$INPATH/inc/external -I${SOLARVER}/$INPATH/inc -I$SRC_ROOT/solenv/inc $SOLARINC"
+SOLARINC="-I. -I${SOLARVER}/$INPATH/inc/external -I${SOLARVER}/$INPATH/inc -I$SRC_ROOT/solenv/inc $SOLARINC"
 AC_SUBST(COM)
 AC_SUBST(CPU)
 AC_SUBST(CPUNAME)