Gentoo-bug: https://bugs.gentoo.org/657820

--- a/configure.ac	2018-03-18 14:51:31.000000000 +0100
+++ b/configure.ac	2018-07-11 23:13:14.901127036 +0200
@@ -119,7 +119,6 @@
 saved_CPPFLAGS="${CPPFLAGS}"
 CPPFLAGS="${GLM_CFLAGS} ${CPPFLAGS}"
 GLM_headers="glm/glm.hpp"
-AS_IF([test "x$enable_tests" == "xyes"], [GLM_headers="$GLM_headers glm/gtx/io.hpp"], [])
 AC_CHECK_HEADERS(
     [$GLM_headers],
     [],