package/netsurf: fix build

package/netsurf: fix build

Fixes:
http://autobuild.buildroot.net/results/a5b/a5bd8969c398fc3101ffaec4aa715a827aec5770/
http://autobuild.buildroot.net/results/441/44112e8ad03f47125bbf4b231d800ebd5beef24b/

After commit 122089ad (package/netsurf: use TMP_PREFIX inside the build
directory), the build fails with:

 COMPILE: src/stylesheet.c
In file included from src/stylesheet.c:12:0:
src/stylesheet.h:14:39: fatal error: libwapcaplet/libwapcaplet.h: No such file or directory
 #include <libwapcaplet/libwapcaplet.h>

The reason is that netsurf installs its internal libraries to TMP_PREFIX
during the build, and uses pkg-config to add the correct include/linker
flags when building/linking the rest.  Unfortunately this fails badly, as we
prefix STAGING_DIR to the paths returned by pkg-config, causing gcc to fail
to find the header / library files.

This worked (by accident) when we pointed TMP_PREFIX to STAGING_DIR/usr, as
STAGING_DIR/usr/include and STAGING_DIR/usr/lib are in the standard
include/library search paths.

Fix it by adding TMP_PREFIX/include and TMP_PREFIX/lib to the
include/library search paths.  We cannot easily add them to CFLAGS/LDFLAGS
as the makefiles do not use override when appending to them, so instead pass
both in CC (which is also used for linking).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>