Pass through extra flags to libcxx build (WebAssembly#322)

This commit passes through `EXTRA_CFLAGS` to the libcxx build, along
with `EXTRA_CXXFLAGS`. This models after the pattern in `wasi-libc` for
passing these flags to thread more flags through.

Author: alexcrichton

Makefile

@@ -187,8 +187,8 @@ build/libcxx.BUILT: build/llvm.BUILT build/compiler-rt.BUILT build/wasi-libc.BUI
 	mkdir -p build/libcxx
 	cd build/libcxx && cmake -G Ninja $(LIBCXX_CMAKE_FLAGS:@PTHREAD@=OFF) \
 		-DCMAKE_SYSROOT=$(BUILD_PREFIX)/share/wasi-sysroot \
-		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP)" \
-		-DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP)" \
+		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) $(EXTRA_CFLAGS)" \
+		-DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) $(EXTRA_CXXFLAGS)" \
 		-DLIBCXX_LIBDIR_SUFFIX=$(ESCAPE_SLASH)/wasm32-wasi \
 		-DLIBCXXABI_LIBDIR_SUFFIX=$(ESCAPE_SLASH)/wasm32-wasi \
 		-DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \
@@ -197,8 +197,8 @@ build/libcxx.BUILT: build/llvm.BUILT build/compiler-rt.BUILT build/wasi-libc.BUI
 	mkdir -p build/libcxx-threads
 	cd build/libcxx-threads && cmake -G Ninja $(LIBCXX_CMAKE_FLAGS:@PTHREAD@=ON) \
 		-DCMAKE_SYSROOT=$(BUILD_PREFIX)/share/wasi-sysroot \
-		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) -pthread" \
-		-DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) -pthread" \
+		-DCMAKE_C_FLAGS="$(DEBUG_PREFIX_MAP) -pthread $(EXTRA_CFLAGS)" \
+		-DCMAKE_CXX_FLAGS="$(DEBUG_PREFIX_MAP) -pthread $(EXTRA_CXXFLAGS)" \
 		-DLIBCXX_LIBDIR_SUFFIX=$(ESCAPE_SLASH)/wasm32-wasi-threads \
 		-DLIBCXXABI_LIBDIR_SUFFIX=$(ESCAPE_SLASH)/wasm32-wasi-threads \
 		-DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \