Building with --without-raster drops CPPFLAGS

[Algunenano]

When I do ./configure I get:

CPPFLAGS:              -I/usr/include   -I/usr/include/libxml2  -I/usr/include/json-c   

If I do ./configure --without-raster I get:

  CPPFLAGS:          

Which later on breaks the build as it can't find json-c headers:

/bin/sh ../libtool --mode=compile gcc  -Wall -Wmissing-prototypes -std=gnu99 -g -O2 -fno-math-errno -fno-signed-zeros  -fPIC -DPIC -c -o lwgeom_median.lo lwgeom_median.c
lwin_geojson.c:36:10: fatal error: json.h: No such file or directory
   36 | #include <json.h>
      |          ^~~~~~~~
compilation terminated.

[Algunenano]

This is my fault, wagyu configure is doing CPPFLAGS_SAVE="$CPPFLAGS_SAVE" to save the flags :(

I'm kind of surprised it didn't break sooner.

[Raul Marin]

In 17566:

Fix wagyu configure dropping CPPFLAGS

Closes #4443