Mesa >= 18.2.5 has newer OpenGL headers from Khronos which change the include guards.
Fixed upstream in https://github.com/nigels-com/glew/commit/7f65a36866f4e24dd1446fe1c9d21424f28bcabd Index: Makefile =================================================================== RCS file: /cvs/ports/graphics/glew/Makefile,v retrieving revision 1.16 diff -u -p -r1.16 Makefile --- Makefile 30 Dec 2016 13:57:51 -0000 1.16 +++ Makefile 28 Jan 2019 07:20:38 -0000 @@ -6,6 +6,7 @@ NOT_FOR_ARCHS= m88k COMMENT= GL Extension Wrangler library DISTNAME= glew-2.0.0 +REVISION= 0 CATEGORIES= graphics MASTER_SITES= ${MASTER_SITE_SOURCEFORGE:=glew/} EXTRACT_SUFX= .tgz Index: patches/patch-include_GL_glew_h =================================================================== RCS file: patches/patch-include_GL_glew_h diff -N patches/patch-include_GL_glew_h --- /dev/null 1 Jan 1970 00:00:00 -0000 +++ patches/patch-include_GL_glew_h 28 Jan 2019 07:20:38 -0000 @@ -0,0 +1,25 @@ +$OpenBSD$ + +7f65a36866f4e24dd1446fe1c9d21424f28bcabd +Fixed compilation with current mesa versions. + +Index: include/GL/glew.h +--- include/GL/glew.h.orig ++++ include/GL/glew.h +@@ -93,7 +93,7 @@ + #if defined(__REGAL_H__) + #error Regal.h included before glew.h + #endif +-#if defined(__glext_h_) || defined(__GLEXT_H_) ++#if defined(__glext_h_) || defined(__GLEXT_H_) || defined(__gl_glext_h_) + #error glext.h included before glew.h + #endif + #if defined(__gl_ATI_h_) +@@ -109,6 +109,7 @@ + #define __X_GL_H + #define __glext_h_ + #define __GLEXT_H_ ++#define __gl_glext_h_ + #define __gl_ATI_h_ + + #if defined(_WIN32)
