build: Use fewer isystem substituions

configure.ac

@@ -75,10 +75,10 @@ AS_IF([test "x$HAVE_GEOS" = "xyes"],
 AS_IF([test "x$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 ,$(GEOS_CFLAGS))'])])
+      [AC_SUBST(GEOS_CFLAGS_SYSTEM, ['$(subst -I,-I,$(GEOS_CFLAGS))'])])
 
 
-AC_SUBST([gdkmm_CFLAGS_SYSTEM], ['$(subst -I,-isystem ,$(gdkmm_CFLAGS))'])
+AC_SUBST([gdkmm_CFLAGS_SYSTEM], ['$(subst -I,-I,$(gdkmm_CFLAGS))'])
 
 AC_SUBST(gerbv_LIBS)
 AC_SUBST(gerbv_CFLAGS)
@@ -91,9 +91,9 @@ CFLAGS+=" ${gerbv_CFLAGS}"
 AX_C_COMPILE_VALUE(GERBV_INTERPOLATION_LINEARx10,NEW_LINEARX10,[#include "gerbv.h"])
 
 # Ignore GTK 2.0 warnings for linux and macos and windows
-AC_SUBST([gerbv_CFLAGS_SYSTEM1], ['$(subst -I/usr/include,-isystem /usr/include,$(gerbv_CFLAGS))'])
-AC_SUBST([gerbv_CFLAGS_SYSTEM2], ['$(subst -I/usr/local/Cellar/gtk+/2.24.33/include,-isystem /usr/local/Cellar/gtk+/2.24.33/include,$(gerbv_CFLAGS_SYSTEM1))'])
-AC_SUBST([gerbv_CFLAGS_SYSTEM], ['$(subst -ID:/a/_temp/msys64/mingw64/include,-isystem D:/a/_temp/msys64/mingw64/include,$(gerbv_CFLAGS_SYSTEM2))'])
+AC_SUBST([gerbv_CFLAGS_SYSTEM1], ['$(subst -I/home/runner/.local/include,-isystem /home/runner/.local/include,$(gerbv_CFLAGS))'])
+AC_SUBST([gerbv_CFLAGS_SYSTEM2], ['$(subst -I/usr/local/Cellar/gtk+/2.24.33/include,-I/usr/local/Cellar/gtk+/2.24.33/include,$(gerbv_CFLAGS_SYSTEM1))'])
+AC_SUBST([gerbv_CFLAGS_SYSTEM], ['$(subst -ID:/a/pcb2gcode/.local/include,-isystem D:/a/pcb2gcode/.local/include,$(gerbv_CFLAGS_SYSTEM2))'])
 
 # Checks for header files.
 AC_HEADER_STDC