Add `--enable-html-docs` to distcheck flags

FIXME: `--enable-pdf-docs` not added because I can't make it pass
distcheck due to quirks of `pdflatex` (see comments in
`doc/Makefile.am`)

Makefile.am

@@ -5,7 +5,7 @@ SUBDIRS = tagmanager scintilla src plugins icons po doc data tests
 AUTOMAKE_OPTIONS =	1.7
 
 ACLOCAL_AMFLAGS = -I m4
-AM_DISTCHECK_CONFIGURE_FLAGS = --enable-api-docs
+AM_DISTCHECK_CONFIGURE_FLAGS = --enable-api-docs --enable-html-docs
 
 WIN32_BUILD_FILES = \
 	geany_private.rc \