meson: make GIT_HTML_PATH configurable

Makefile-based builds can configure Git's internal HTML_PATH by defining
htmldir, which is useful for packagers that put documentation in
different locations. Gentoo, for example, uses version-suffixed
directories like ${prefix}/share/doc/git-2.51 and puts the HTML
documentation in an 'html' subdirectory of the same.

Propagate the same configuration knob to Meson-based builds so that
"git --html-path" on such systems can be configured to output the
correct directory.

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

meson.build

@@ -768,13 +768,18 @@ if test_output_directory == ''
   test_output_directory = meson.project_build_root() / 'test-output'
 endif
 
+htmldir = get_option('htmldir')
+if htmldir == ''
+  htmldir = get_option('datadir') / 'doc/git-doc'
+endif
+
 # These variables are used for building libgit.a.
 libgit_c_args = [
   '-DBINDIR="' + get_option('bindir') + '"',
   '-DDEFAULT_GIT_TEMPLATE_DIR="' + get_option('datadir') / 'git-core/templates' + '"',
   '-DFALLBACK_RUNTIME_PREFIX="' + get_option('prefix') + '"',
   '-DGIT_HOST_CPU="' + host_machine.cpu_family() + '"',
-  '-DGIT_HTML_PATH="' + get_option('datadir') / 'doc/git-doc"',
+  '-DGIT_HTML_PATH="' + htmldir + '"',
   '-DGIT_INFO_PATH="' + get_option('infodir') + '"',
   '-DGIT_LOCALE_PATH="' + get_option('localedir') + '"',
   '-DGIT_MAN_PATH="' + get_option('mandir') + '"',