glsl/build: Build glcpp via the glsl Makefile

Removing the subdirectory recursion provides a small speed up.

Tested-by: Andreas Boll <andreas.boll.dev@gmail.com>
diff --git a/doxygen/glsl.doxy b/doxygen/glsl.doxy
index aa907ae..9915ba2 100644
--- a/doxygen/glsl.doxy
+++ b/doxygen/glsl.doxy
@@ -11,8 +11,8 @@
 #---------------------------------------------------------------------------
 INPUT                  = ../src/glsl/
 RECURSIVE              = NO
-EXCLUDE                = ../src/glsl/glsl_lexer.cc \
-                         ../src/glsl/glsl_parser.cc \
+EXCLUDE                = ../src/glsl/glsl_lexer.cpp \
+                         ../src/glsl/glsl_parser.cpp \
                          ../src/glsl/glsl_parser.h
 EXCLUDE_PATTERNS       =
 #---------------------------------------------------------------------------