PortAudio: Enable build on macOS/arm64

Quote from portaudio/config.log:
...
configure:3194: checking whether the C compiler works
configure:3216: gcc  -I/opt/homebrew/opt/zlib/include portaudio/lib/.libs/libportaudio.a -framework CoreServices -framework CoreFoundation -framework AudioUnit -framewo    rk AudioToolbox -framework CoreAudio -lpthread -lm mini-gdbstub/build/libgdbstub.a conftest.c  >&5
clang: error: no such file or directory: 'portaudio/lib/.libs/libportaudio.a'
...
configure:3265: error: C compiler cannot create executables
...

The ./configure script fails on macOS because LDFLAGS includes static
libraries (libportaudio.a) that have not yet been built. This causes the
compiler check to fail with "C compiler cannot create executables".

This patch unsets LDFLAGS during the configure step to allow the test
program to compile and link successfully, enabling builds on macOS.

Related: #76

Author: ChinYikMing

Makefile

@@ -100,7 +100,7 @@ ifeq ($(call has, VIRTIOSND), 1)
 portaudio/Makefile:
 	git submodule update --init portaudio
 $(PORTAUDIOLIB): portaudio/Makefile
-	cd $(dir $<) && ./configure --without-sndio
+	cd $(dir $<) && LDFLAGS="" ./configure --without-sndio
 	$(MAKE) -C $(dir $<)
 main.o: $(PORTAUDIOLIB)
 
@@ -168,12 +168,12 @@ DTC ?= dtc
 E :=
 S := $E $E
 
-# During boot process, he emulator manually manages the growth of ticks to 
-# suppress RCU CPU stall warnings. Thus, we need an target time to set the 
+# During boot process, he emulator manually manages the growth of ticks to
+# suppress RCU CPU stall warnings. Thus, we need an target time to set the
 # increment of ticks. According to Using RCU’s CPU Stall Detector[1], the
-# grace period for RCU CPU stalls is typically set to 21 seconds. 
-# By dividing this value by two as the expected completion time, we can 
-# provide a sufficient buffer to reduce the impact of errors and avoid 
+# grace period for RCU CPU stalls is typically set to 21 seconds.
+# By dividing this value by two as the expected completion time, we can
+# provide a sufficient buffer to reduce the impact of errors and avoid
 # RCU CPU stall warnings.
 # [1] docs.kernel.org/RCU/stallwarn.html#config-rcu-cpu-stall-timeout
 CFLAGS += -D SEMU_BOOT_TARGET_TIME=10
@@ -213,6 +213,6 @@ distclean: clean
 	$(Q)$(RM) riscv-harts.dtsi
 	$(Q)$(RM) minimal.dtb
 	$(Q)$(RM) Image rootfs.cpio
-	$(Q)$(RM) ext4.img 
+	$(Q)$(RM) ext4.img
 
 -include $(deps)