fix builds that use older clang versions

[wuputah]

pass PG_CPPFLAGS and PG_CXXFLAGS to COMPILE.cxx.bc like we did for COMPILE.cc.bc

this fixes the build on ubuntu:22.04

[wuputah]

Another cause looks like our %.bc: %.cpp rule isn't being picked up as it's defined before the include. However, if moved after the include, that rule doesn't seem to work.

g++ -Wno-ignored-attributes -Wno-register -fno-strict-aliasing -fwrapv -Xclang -no-opaque-pointers -O2  -Wall -Wpointer-arith -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wcast-function-type -Wshadow=compatible-local -Wformat-security -fno-strict-aliasing -fwrapv -moutline-atomics -g -g -O2 -flto=auto -ffat-lto-objects -flto=auto -ffat-lto-objects -fstack-protector-strong -Wformat -Werror=format-security -std=c++17 -Wno-sign-compare -Wno-register  -fPIC -fvisibility=hidden -fvisibility-inlines-hidden -Iinclude -Ithird_party/duckdb/src/include -Ithird_party/duckdb/third_party/re2 -std=c++17 -Wno-sign-compare -Wno-register  -I/usr/include/postgresql/16/server -emit-llvm -c -Wl,-rpath,/usr/lib/postgresql/16/lib/ -lpq -Lthird_party/duckdb/build/release/src -L/usr/lib/postgresql/16/lib -lduckdb -lstdc++ -llz4  -I -o src/pgduckdb.bc src/pgduckdb.cpp
g++: error: unrecognized command-line option '-Xclang'
g++: error: unrecognized command-line option '-no-opaque-pointers'

[Y--]

LGTM if it works :-)

[wuputah]

Build is failing for the -Wall issue I mentioned in #240 -- now that duckdb is being recompiled after switching to 24.04.