Specify %option prefix="glcpp_" in the source code, not the Makefile.
diff --git a/glcpp/Makefile b/glcpp/Makefile
index 3fb44ac..1578a8e 100644
--- a/glcpp/Makefile
+++ b/glcpp/Makefile
@@ -13,7 +13,7 @@
bison --debug --defines=$*.h --output=$*.c $^
%.c: %.l
- flex --prefix=glcpp_ --outfile=$@ $<
+ flex --outfile=$@ $<
glcpp-lex.c: glcpp-parse.h
diff --git a/glcpp/glcpp-lex.l b/glcpp/glcpp-lex.l
index 0d9a754..cc5f28f 100644
--- a/glcpp/glcpp-lex.l
+++ b/glcpp/glcpp-lex.l
@@ -31,6 +31,7 @@
%option reentrant noyywrap
%option extra-type="glcpp_parser_t *"
+%option prefix="glcpp_"
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