Use isystem for geos headers

Because the geos and boost headers are in the same place, this is necessary so that using geos won't cause warnings from boost.

Makefile.am

@@ -68,7 +68,7 @@ ACLOCAL_AMFLAGS = -I m4
 GIT_VERSION = `git describe --dirty --always --tags`
 GERBV_VERSION = `pkg-config --modversion libgerbv`
 
-AM_CPPFLAGS = $(BOOST_CPPFLAGS_SYSTEM) $(glibmm_CFLAGS) $(gdkmm_CFLAGS_SYSTEM) $(gerbv_CFLAGS_SYSTEM) $(CODE_COVERAGE_CPPFLAGS) -DGIT_VERSION=\"$(GIT_VERSION)\" -Wall -Wpedantic -Wextra $(pcb2gcode_CPPFLAGS_EXTRA) $(GEOS_CFLAGS) $(GEOS_EXTRA)
+AM_CPPFLAGS = $(BOOST_CPPFLAGS_SYSTEM) $(glibmm_CFLAGS) $(gdkmm_CFLAGS_SYSTEM) $(gerbv_CFLAGS_SYSTEM) $(CODE_COVERAGE_CPPFLAGS) -DGIT_VERSION=\"$(GIT_VERSION)\" -Wall -Wpedantic -Wextra $(pcb2gcode_CPPFLAGS_EXTRA) $(GEOS_CFLAGS_SYSTEM) $(GEOS_EXTRA)
 AM_CXXFLAGS = $(CODE_COVERAGE_CXXFLAGS) -DGIT_VERSION=\"$(GIT_VERSION)\" -DGERBV_VERSION=\"$(GERBV_VERSION)\"
 AM_LDFLAGS = $(BOOST_PROGRAM_OPTIONS_LDFLAGS) $(pcb2gcode_LDFLAGS_EXTRA)
 LIBS = $(gerbv_LIBS) $(BOOST_PROGRAM_OPTIONS_LIBS) $(CODE_COVERAGE_LIBS) $(GEOS_CC_LIBS)

configure.ac

@@ -64,6 +64,8 @@ AS_IF([test "x$HAVE_GEOS" = "xyes"],
       [AC_MSG_NOTICE([Didn't find geos, we'll use boost geometry for all geometry functions.  If you see inaccuracy, try geos.])])
 AS_IF([test "x$HAVE_GEOS" = "xyes"],
       [AC_SUBST(GEOS_EXTRA, ["-DUSE_UNSTABLE_GEOS_CPP_API -DGEOS_VERSION=\\\"\$(GEOS_VERSION)\\\""])])
+AS_IF([test "x$HAVE_GEOS" = "xyes"],
+      [AC_SUBST(GEOS_CFLAGS_SYSTEM, ['$(subst -I,-isystem ,$(GEOS_CFLAGS))'])])
 
 
 AC_SUBST([gdkmm_CFLAGS_SYSTEM], ['$(subst -I,-isystem ,$(gdkmm_CFLAGS))'])