diff --git a/build/Linux/Makefile b/build/Linux/Makefile
index d08b6df..01e80cf 100755
--- a/build/Linux/Makefile
+++ b/build/Linux/Makefile
@@ -54,7 +54,7 @@ docdir = $(datarootdir)/doc/epanet
 
 # Compiler and flags
 CC = gcc
-CFLAGS = -g -O3 -fPIC
+CFLAGS = -g -O3 -fPIC -std=c99
 CPPFLAGS = -I $(epanetincludedir)
 LDFLAGS = -L . -Wl,-rpath,$(libdir) -lm
 
diff --git a/build/Linux/Makefile_Rev b/build/Linux/Makefile_Rev
index 488e471..362725d 100755
--- a/build/Linux/Makefile_Rev
+++ b/build/Linux/Makefile_Rev
@@ -57,7 +57,7 @@ docdir = $(datarootdir)/doc/epanet
 
 # Compiler and flags
 CC = gcc
-CFLAGS = -g -O3 -fPIC
+CFLAGS = -g -O3 -fPIC -std=c99
 CPPFLAGS = -I $(epanetincludedir)
 LDFLAGS = -L . -Wl,-rpath,$(libdir) -lm