FTGL misdetected by ./configure

[claudeha]

What happens:

Regardless of whether libftgl2 and/or libftgl-dev are installed or not on Debian Buster, I get this from ./configure:

 used optional libraries:

  font-rendering         : FTGL
     default font        : 

which leads to this if libftgl-dev is not installed:

Making all in Base
make[3]: Entering directory '/home/claude/code/github.com/umlaeute/Gem/src/Base'
/bin/bash ../../libtool  --tag=CXX   --mode=compile g++ -DHAVE_CONFIG_H -I. -I../../src -I../../src/Gem  -I../../src -DGEM_INTERNAL   -DPD -DPD -I/usr/include/pd       -g -O2 -freg-struct-return -O3 -falign-loops -falign-functions -falign-jumps -funroll-loops -ffast-math -mmmx -MT libBase_la-TextBaseFTGL.lo -MD -MP -MF .deps/libBase_la-TextBaseFTGL.Tpo -c -o libBase_la-TextBaseFTGL.lo `test -f 'TextBaseFTGL.cpp' || echo './'`TextBaseFTGL.cpp
UNUSED SOURCES in .: libBase_la-GemGluObj.o libBase_la-GemPixObj.o libBase_la-GemBase.o libBase_la-GemShape.o libBase_la-GemPixDualObj.o libBase_la-CPPExtern.o libBase_la-GemVertex.o libBase_la-GemWindow.o libBase_la-GemContext.o libBase_la-GemPathBase.o
libtool: compile:  g++ -DHAVE_CONFIG_H -I. -I../../src -I../../src/Gem -I../../src -DGEM_INTERNAL -DPD -DPD -I/usr/include/pd -g -O2 -freg-struct-return -O3 -falign-loops -falign-functions -falign-jumps -funroll-loops -ffast-math -mmmx -MT libBase_la-TextBaseFTGL.lo -MD -MP -MF .deps/libBase_la-TextBaseFTGL.Tpo -c TextBaseFTGL.cpp  -fPIC -DPIC -o .libs/libBase_la-TextBaseFTGL.o
In file included from TextBaseFTGL.cpp:24:
TextBase.h:39:12: fatal error: FTFont.h: No such file or directory
 #  include "FTFont.h"
            ^~~~~~~~~~
compilation terminated.

To compile without FTGL it is necessary to use ./configure --without-ftgl. This allows build to succeed.

Expected behaviour:

./configure automatically detects presence or absence of FTGL and allows build to succeed in all circumstances (without FTGL support if it isn't detected).