Use -std=gnu17 instead of -std=c17 for GCC and test (#7588)

Author: mcspr

platform.txt

@@ -55,7 +55,7 @@ compiler.libc.path={runtime.platform.path}/tools/sdk/libc/xtensa-lx106-elf
 compiler.cpreprocessor.flags=-D__ets__ -DICACHE_FLASH -U__STRICT_ANSI__ "-I{compiler.sdk.path}/include" "-I{compiler.sdk.path}/{build.lwip_include}" "-I{compiler.libc.path}/include" "-I{build.path}/core"
 
 compiler.c.cmd=xtensa-lx106-elf-gcc
-compiler.c.flags=-c {compiler.warning_flags} -std=c17 {build.stacksmash_flags} -Os -g -Wpointer-arith -Wno-implicit-function-declaration -Wl,-EL -fno-inline-functions -nostdlib -mlongcalls -mtext-section-literals -falign-functions=4 -MMD -ffunction-sections -fdata-sections {build.exception_flags} {build.sslflags}
+compiler.c.flags=-c {compiler.warning_flags} -std=gnu17 {build.stacksmash_flags} -Os -g -Wpointer-arith -Wno-implicit-function-declaration -Wl,-EL -fno-inline-functions -nostdlib -mlongcalls -mtext-section-literals -falign-functions=4 -MMD -ffunction-sections -fdata-sections {build.exception_flags} {build.sslflags}
 
 compiler.S.cmd=xtensa-lx106-elf-gcc
 compiler.S.flags=-c -g -x assembler-with-cpp -MMD -mlongcalls

tools/platformio-build.py

@@ -54,7 +54,7 @@ def scons_patched_match_splitext(path, suffixes=None):
     ASFLAGS=["-x", "assembler-with-cpp"],
 
     CFLAGS=[
-        "-std=c17",
+        "-std=gnu17",
         "-Wpointer-arith",
         "-Wno-implicit-function-declaration",
         "-Wl,-EL",