diff --git a/README.md b/README.md
index f9d3c59..8659c6a 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@ Configure with cross-compiler in `$PATH` to make without it in `$PATH`:
   AR="$(which i686-elf-ar)" \
   CC="$(which i686-elf-gcc)" \
   RANLIB="$(which i686-elf-ranlib)" \
-  CFLAGS='-std=gnu99 -ffreestanding -nostdinc -nostdlib -fno-builtin -fno-stack-protector -Wall -Wextra'
+  CFLAGS='-ffreestanding -nostdinc -nostdlib -fno-builtin -fno-stack-protector -Wall -Wextra'
 ```
 
 Check if compilation targets i386: `objdump -d src/arch/i386.o`.
diff --git a/configure.ac b/configure.ac
index 06631b0..d40ce8b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -20,6 +20,7 @@ AC_LANG([C])
 AM_PROG_AR
 AM_PROG_AS
 AC_PROG_CC
+AC_PROG_CC_C99
 AC_PROG_RANLIB
 
 AC_OUTPUT