fltk: add explicit dependency on gl and a variant to activate it (#22761)

* The fltk package can build libraries with opengl support. By default, the configure script looks for opengl headers in the sytem include paths. If 'devel' packages have not been installed on the system it omits the 'ftlk_gl.so' library. This can break packages like 'octave' which expects 'fltk' to have opengl support and looks for the library 'fltk_gl'.

Make opengl support explicit in fltk by adding a dependency on 'gl' and adding a new variant of the same name 'gl' (default On).

With these modifications 'fltk_gl' and 'octave' build successfully on CentOS8.

The default behavior is to always enable opengl.

https://www.fltk.org/doc-1.3/intro.html

var/spack/repos/builtin/packages/fltk/package.py

@@ -38,6 +38,12 @@ class Fltk(Package):
     variant('shared', default=True,
             description='Enables the build of shared libraries')
 
+    variant('gl', default=True,
+            description='Enables opengl support')
+
+    # variant dependencies
+    depends_on('gl', when='+gl')
+
     def install(self, spec, prefix):
         options = ['--prefix=%s' % prefix,
                    '--enable-localjpeg',
@@ -47,6 +53,9 @@ def install(self, spec, prefix):
         if '+shared' in spec:
             options.append('--enable-shared')
 
+        if '~gl' in spec:
+            options.append('--disable-gl')
+
         # FLTK needs to be built in-source
         configure(*options)
         make()