gnu source: honoring libc requirements (#8147)

Author: d-a-v

platform.txt

@@ -56,7 +56,7 @@ compiler.path={runtime.tools.xtensa-lx106-elf-gcc.path}/bin/
 compiler.sdk.path={runtime.platform.path}/tools/sdk
 
 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.cpreprocessor.flags=-D__ets__ -DICACHE_FLASH -U__STRICT_ANSI__ -D_GNU_SOURCE "-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=gnu17 {build.stacksmash_flags} -Os -g -free -fipa-pta -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} {build.mmuflags} {build.non32xferflags}

tools/platformio-build.py

@@ -73,6 +73,7 @@ def scons_patched_match_splitext(path, suffixes=None):
         "-mtext-section-literals",
         "-falign-functions=4",
         "-U__STRICT_ANSI__",
+        "-D_GNU_SOURCE",
         "-ffunction-sections",
         "-fdata-sections",
         "-Wall",