Use correct syntax for the -dylib_file flag (no space after colon).
Fixes configure failure on macOS Big Sur and later:

configure: error: GL library could not be found, please specify its location with --with-gl-lib.

config.log contains:

configure:17551: /usr/bin/clang -o conftest -pipe -Os -DGL_SILENCE_DEPRECATION -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX11.0.sdk -arch x86_64  -L/opt/local/lib -Wl,-headerpad_max_install_names -Wl,-syslibroot,/Library/Developer/CommandLineTools/SDKs/MacOSX11.0.sdk -arch x86_64 conftest.c -Xlinker -framework -Xlinker OpenGL -dylib_file /System/Library/Frameworks/OpenGL.framework/Versions/A/Libraries/libGL.dylib: /System/Library/Frameworks/OpenGL.framework/Versions/A/Libraries/libGL.dylib >&5
clang: error: no such file or directory: '/System/Library/Frameworks/OpenGL.framework/Versions/A/Libraries/libGL.dylib'

https://github.com/frankheckenbach/ftgl/pull/14
--- m4/gl.m4.orig	2019-02-07 15:39:58.000000000 -0600
+++ m4/gl.m4	2021-02-10 23:23:19.000000000 -0600
@@ -50,7 +50,7 @@
 # -Xlinker is used because libtool is busted prior to 1.6 wrt frameworks
 AC_TRY_LINK([#include <OpenGL/gl.h>], [glBegin(GL_POINTS)],
     [GL_DYLIB="/System/Library/Frameworks/OpenGL.framework/Versions/A/Libraries/libGL.dylib"
-     FRAMEWORK_OPENGL="-Xlinker -framework -Xlinker OpenGL -dylib_file $GL_DYLIB: $GL_DYLIB"
+     FRAMEWORK_OPENGL="-Xlinker -framework -Xlinker OpenGL -dylib_file $GL_DYLIB:$GL_DYLIB"
      ac_cv_search_glBegin="$FRAMEWORK_OPENGL"
      AC_MSG_RESULT(yes)],
     [AC_MSG_RESULT(no)])