diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6687b56..c19d29a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -66,6 +66,13 @@
     CACHE STRING "Flags used by the C++ compiler during AddressSanitizer builds."
     FORCE)
 
+function(check_and_add_compiler_flag flag variable)
+    check_c_compiler_flag(${flag} ${variable})
+    if (${variable})
+        add_compile_options(${flag})
+    endif()
+endfunction()
+
 if (MSVC)
     add_definitions(-D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE -D_WINSOCK_DEPRECATED_NO_WARNINGS)
     if (USE_WERROR)
@@ -96,11 +103,9 @@
     endif()
 else ()
     add_compile_options(-Wall -Wuninitialized)
-    check_c_compiler_flag(-Wno-stringop-truncation NoStringOpTruncation)
-
-    if (${NoStringOpTruncation})
-        add_compile_options(-Wno-stringop-truncation)
-    endif()
+    check_and_add_compiler_flag(-Wno-stringop-truncation NoStringOpTruncation)
+    check_and_add_compiler_flag(-Wstrict-prototypes StrictPrototypes)
+    check_and_add_compiler_flag(-Wold-style-definition OldStyleDefinition)
     # We are not ready for this
     #add_compile_options(-Wconversion -Wno-sign-conversion -Wsign-compare)
     if (USE_WERROR)
diff --git a/tests/unit_tests/openvpn/test_cryptoapi.c b/tests/unit_tests/openvpn/test_cryptoapi.c
index 98102ef..478995f 100644
--- a/tests/unit_tests/openvpn/test_cryptoapi.c
+++ b/tests/unit_tests/openvpn/test_cryptoapi.c
@@ -116,7 +116,7 @@
 
 /* Fill-in certs[] array */
 void
-init_cert_data()
+init_cert_data(void)
 {
     struct test_cert certs_local[] = {
         {cert1,  key1,  cname1,  "OVPN TEST CA1",  "OVPN Test Cert 1",  hash1,  1},