patch 9.1.1644: configure: doesn't separate CPPFLAGS and CFLAGS

patch 9.1.1644: configure: doesn't separate CPPFLAGS and CFLAGS

Problem:  configure: doesn't separate CPPFLAGS and CFLAGS
Solution: Split CPPFLAGS and CFLAGS for pkg-config
          (Damien Lejay)

Previously, all flags returned by pkg-config --cflags were dumped into
CFLAGS, mixing include paths with compiler options. This commit uses
--cflags-only-I and --cflags-only-other to properly separate include
flags into CPPFLAGS and keep compiler flags in CFLAGS.

closes: #18019

Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: Christian Brabandt <cb@256bit.org>