doc: make PDF documentation optional

While building any documentation requires xsltproc, only the PDF
documentation requires fop, which is dependency-heavy, and not always
available (e.g. it is not included in RHEL).  This allows the HTML
documentation to be built while skipping the PDFs.

doc/guide-meson.build

@@ -56,6 +56,7 @@ custom_target(
 )
 
 
+if prog_fop.found()
   fop = custom_target(
     input: xml,
     output: name + '.fop',
@@ -85,6 +86,7 @@ custom_target(
     install_dir: pdfdir,
     install_tag: 'doc',
   )
+endif
 
 
 subdir('html')

meson.build

@@ -502,7 +502,7 @@ if enable_docs
   endif
   prog_fop = find_program(
     'fop',
-    required: feature_docs,
+    required: false,
     disabler: true,
   )