configure.ac: Check for GLM in standard include path

Looks like Arch Linux stopped providing pkg-config file for glm. Since
this is mostly a header-only library we can just check if the headers
exist.

configure.ac

@@ -1185,6 +1185,10 @@ elif test $cross_compile = no; then
         if test $found_glm_hpp = yes; then
                 glm=yes
                 GLM_INC=-I$GLM_DIR
+        else
+                AC_LANG_PUSH(C++)
+                AC_CHECK_HEADER([glm/glm.hpp], [glm=yes], [glm=no])
+                AC_LANG_POP(C++)
         fi
 fi