package/pugixml: fix configuration options

Commit upstream 1c5a0bb32583fd294022e68e66b541bf6ff71a67
"✨

 Update CMake to modern approach" removed all feature
toogles, so starting from release v1.11 any flag passed in BUILD_DEFINES
was completely ignored during the build.
Version 1.11.4 was in use in buildroot since February 2021.

Commit upstream 0f1e75a902ef1751dd63a67fe223b5e8daf4c7f1
"Re-introduced the custom build defines" added PUGIXML_BUILD_DEFINES to
release v1.12, but BUILD_DEFINES is still completely ignored during the
build.
Version 1.12.1 is in use in buildroot since March 2022.

So switch the config options to use PUGIXML_BUILD_DEFINES instead of
BUILD_DEFINES.

Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Theo Debrouwere <t.debrouwere@televic.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>