diff --git a/como-win32-tools.jam b/como-win32-tools.jam
index f759378c7..0f2eec8bf 100644
--- a/como-win32-tools.jam
+++ b/como-win32-tools.jam
@@ -109,7 +109,7 @@ actions como-Cc-action
     $(COMO_BACKEND_LIB_SETUP)
     $(COMO_PATH_SETUP)
     $(COMO_BASE_SETUP)
-    $(COMO_BIN_DIRECTORY)como -c -e5 --no_version --display_error_number --diag_suppress=9,21,161,748,940,962 -U$(UNDEFS) -D$(DEFINES) $(WARN) $(CFLAGS) -I"$(HDRS)" -I"$(STDHDRS)" -o "$(<:D=)" "$(>)"
+    $(COMO_BIN_DIRECTORY)como -c --c99 -e5 --no_version --display_error_number --diag_suppress=9,21,161,748,940,962 -U$(UNDEFS) -D$(DEFINES) $(WARN) $(CFLAGS) -I"$(HDRS)" -I"$(STDHDRS)" -o "$(<:D=)" "$(>)"
 }
 
 #### C++ ####
diff --git a/v1/como-win32-tools.jam b/v1/como-win32-tools.jam
index f759378c7..0f2eec8bf 100644
--- a/v1/como-win32-tools.jam
+++ b/v1/como-win32-tools.jam
@@ -109,7 +109,7 @@ actions como-Cc-action
     $(COMO_BACKEND_LIB_SETUP)
     $(COMO_PATH_SETUP)
     $(COMO_BASE_SETUP)
-    $(COMO_BIN_DIRECTORY)como -c -e5 --no_version --display_error_number --diag_suppress=9,21,161,748,940,962 -U$(UNDEFS) -D$(DEFINES) $(WARN) $(CFLAGS) -I"$(HDRS)" -I"$(STDHDRS)" -o "$(<:D=)" "$(>)"
+    $(COMO_BIN_DIRECTORY)como -c --c99 -e5 --no_version --display_error_number --diag_suppress=9,21,161,748,940,962 -U$(UNDEFS) -D$(DEFINES) $(WARN) $(CFLAGS) -I"$(HDRS)" -I"$(STDHDRS)" -o "$(<:D=)" "$(>)"
 }
 
 #### C++ ####