build: Add a standard way to use GNU extensions from libc, since we have a general configured newlib in the toolchain

make/project.mk

@@ -427,6 +427,9 @@ ifdef CONFIG_OPTIMIZATION_ASSERTIONS_DISABLED
 CPPFLAGS += -DNDEBUG
 endif
 
+# IDF uses some GNU extension from libc
+CPPFLAGS += -D_GNU_SOURCE
+
 # Enable generation of debugging symbols
 # (we generate even in Release mode, as this has no impact on final binary size.)
 DEBUG_FLAGS ?= -ggdb

tools/cmake/idf_functions.cmake

@@ -107,6 +107,9 @@ function(idf_set_global_compile_options)
     list(APPEND c_compile_options "-std=gnu99")
     list(APPEND cxx_compile_options "-std=gnu++11" "-fno-rtti")
 
+    # IDF uses some GNU extension from libc
+    list(APPEND compile_definitions "_GNU_SOURCE")
+
     if(CONFIG_CXX_EXCEPTIONS)
         list(APPEND cxx_compile_options "-fexceptions")
     else()