diff --git a/CMakeLists.txt b/CMakeLists.txt
index f8859a319..210688bc5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,7 +41,6 @@ else()
     # does not recognize this yet.
     add_compile_options(-Wno-gnu-zero-variadic-macro-arguments)
   endif()
-  add_definitions(-D_POSIX_C_SOURCE=200809L)
   if(SANITIZERS)
     set(SAN_FLAGS -fsanitize=shift -fsanitize=integer-divide-by-zero
                   -fsanitize=unreachable -fsanitize=vla-bound
diff --git a/Makefile b/Makefile
index 0afec4348..d8a162101 100644
--- a/Makefile
+++ b/Makefile
@@ -32,8 +32,7 @@ WARNFLAGS	:= -Wall -pedantic -Wno-unknown-warning-option \
 # Overridable CXXFLAGS
 CXXFLAGS	?= -O3 -flto -DNDEBUG
 # Non-overridable CXXFLAGS
-REALCXXFLAGS	:= ${CXXFLAGS} ${WARNFLAGS} -std=c++2a -I include \
-		   -D_POSIX_C_SOURCE=200809L -fno-exceptions -fno-rtti
+REALCXXFLAGS	:= ${CXXFLAGS} ${WARNFLAGS} -std=c++2a -I include -fno-exceptions -fno-rtti
 # Overridable LDFLAGS
 LDFLAGS		?=
 # Non-overridable LDFLAGS