Properly quote paths for GCC warnings files

[earlephilhower]

Fixes issue identified by @jjsuwa-sys3175 in
46190b6#r69649862

Tested by adding a space to my git repo's path:

"/home/earle/Arduino/hardware/esp8266com space/esp8266/tools/xtensa-lx106-elf/bin/xtensa-lx106-elf-g++" -D__ets__ -DICACHE_FLASH -U__STRICT_ANSI__ -D_GNU_SOURCE -DESP8266 "-I/home/earle/Arduino/hardware/esp8266com space/esp8266/tools/sdk/include" "-I/home/earle/Arduino/hardware/esp8266com space/esp8266/tools/sdk/lwip2/include" "-I/home/earle/Arduino/hardware/esp8266com space/esp8266/tools/sdk/libc/xtensa-lx106-elf/include" -I/tmp/arduino_build_295618/core -c "@/home/earle/Arduino/hardware/esp8266com space/esp8266/tools/warnings/none-g++" -Os -g -free -fipa-pta -Werror=return-type -mlongcalls -mtext-section-literals -fno-rtti -falign-functions=4 -std=gnu++17 -MMD -ffunction-sections -fdata-sections -fno-exceptions -DMMU_IRAM_SIZE=0x8000 -DMMU_ICACHE_SIZE=0x8000 -DNONOSDK22x_190703=1 -DF_CPU=80000000L -DLWIP_OPEN_SRC -DTCP_MSS=536 -DLWIP_FEATURES=1 -DLWIP_IPV6=0 -DARDUINO=10816 -DARDUINO_ESP8266_WEMOS_D1MINI -DARDUINO_ARCH_ESP8266 "-DARDUINO_BOARD=\"ESP8266_WEMOS_D1MINI\"" -DFLASHMODE_DIO "-DSTASSID=\"XXXX\"" "-DSTAPSK=\"XXXX\"" "-I/home/earle/Arduino/hardware/esp8266com space/esp8266/cores/esp8266" "-I/home/earle/Arduino/hardware/esp8266com space/esp8266/variants/d1_mini" "/home/earle/Arduino/hardware/esp8266com space/esp8266/cores/esp8266/spiffs/spiffs_check.cpp" -o /tmp/arduino_build_295618/core/spiffs/spiffs_check.cpp.o

[jjsuwa-sys3175]

LGTM, and thanks for QFEing :)