package/netsurf: change how CFLAGS/LDFLAGS are passed

Fixes:
http://autobuild.buildroot.net/results/eeb2863c6237aac8428e49a5ee514d43088b0fb8
http://autobuild.buildroot.net/results/f938fd1515f1d6e11b57aa6e314135789da52a44



In commit 6da049f8 ("package/netsurf:
fix build"), the CC variable passed to netsurf's build system was
extended to pass some special -I and -L options needed for netsurf to
find its own headers/libraries.

Unfortunately, on some systems (including mine), it breaks the build,
due to:

  toolpath_ := $(shell /bin/which $(CC__))

when $(CC__) contains some -I/-L options, they are considered to be
options "to which", which causes the funny:

/usr/bin/make install --directory=libnslog HOST=arm-buildroot-linux-uclibcgnueabi PREFIX=/home/thomas/projets/outputs/shared-netsurf/build/netsurf-3.8/tmpusr Q=@ WARNFLAGS='-Wall -W -Wno-error' DESTDIR=
make[3]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.
/bin/which: invalid option -- 'I'
/bin/which: invalid option -- '/'
/bin/which: invalid option -- 'h'
/bin/which: invalid option -- 'o'
/bin/which: invalid option -- 'm'
/bin/which: invalid option -- 'e'
/bin/which: invalid option -- '/'
/bin/which: invalid option -- 't'
/bin/which: invalid option -- 'h'
/bin/which: invalid option -- 'o'
/bin/which: invalid option -- 'm'
/bin/which: invalid option -- 's'
/bin/which: invalid option -- '/'
[...]
/bin/which: invalid option -- 'l'
/bin/which: invalid option -- 'b'
/bin/which: --read-alias, -i: Warning: stdin is a tty.

and the build simply hangs.

We cannot pass CFLAGS/LDFLAGS as make options, as they would override
the CFLAGS definitions in netsurf Makefiles. However, those Makefiles
use the construct:

CFLAGS := $(CFLAGS) -more-flags

so by passing CFLAGS and LDFLAGS through the make environment, which
can achieve our goal.

It is worth mentioning that it remains very fragile, because
CFLAGS/LDFLAGS are used both for building target objects but also some
host tools. The netsurf build system is really not good.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>