Add dynamic path resolution for Windows

[bact]

Find .dll for Windows to fix #7 and fix #17

Add win_inst_dir to handle path resolution for Windows.

Uses standard Windows APIs (partly based on #12 by @fanc999 and from @thep 's suggestion in #36 (comment)):

• GetModuleHandleExW: To get the DLL module handle.
• GetModuleFileNameW: To retrieve the DLL's installation path (handle Unicode characters)

Tested with mingw-w64 (cross-compiled), macOS, Ubuntu.

--

Path resolution flow:

#	Step	Function	Output
1	Get DLL module handle	GetModuleHandleExW (in win_inst_dir)	DLL handle, or NULL on failure
2	Get DLL full path	GetModuleFileNameW (in win_inst_dir)	DLL path = C:\App\bin\libthai.dll or C:\App2\libthai.dll, or NULL on failure
3	Get install path	win_find_inst_dir	Install dir = C:\App (remove "bin") or C:\App2 (no "bin" found, just use the full dir), or NULL on malloc failure
4	Guess dictionary path	wfull_path (in brk_load_default_dict)	Dict path = C:\App\share\libthai\thbrk.tri or C:\App2\share\libthai\thbrk.tri, or NULL on malloc failure
5	Open guessed path	_wfopen (in brk_load_default_dict)	File handle, or NULL if no file exists there
6	Parse dictionary	trie_fread (in brk_load_default_dict)	Trie *dict_trie, or NULL if the file is bad
7	Fall back on any failure in 1-6	trie_new_from_file(DICT_DIR "/" DICT_NAME ".tri") (in brk_load_default_dict)	Likely fails on Windows, but will bring us to regular fallback path before this patch

[bact]

Btw, not sure if it is ok to add th_get_win32_installdir to priv-utils.h.

I can create a separate file (dir-utils.h?), if priv-utils.h is not a good location to host path utils.

[bact]

Note that #12 and #30 also solve the same issue that this PR is trying to solve.

But this PR only focuses on path resolution, not an entirely new build process. Comparisons:

• Improve Windows support and add Visual Studio NMake Makefiles #12
  • Introduces NMake
  • Fixes path resolution at compile time (similar to this PR)
• Add meson build #30
  • Introduces Meson
  • Avoid manual path resolution at compile time as it let Meson manages that

[bact]

Updated the install dir finding algorithm per reviewer suggestion, with some adaptation for wide-char path in Windows. (see the flow table in the updated PR description for general idea; Step 3-4 are heuristic)

@thep the libthai.c is now crowded with functions for Windows, do you have concerns on that? Or should we move them to a location specifically for utility functions?

[thep]

@thep the libthai.c is now crowded with functions for Windows, do you have concerns on that? Or should we move them to a location specifically for utility functions?

Yes, it should be good to have the Windows-specific stuffs in a separate file.

Meanwhile, the static pointer cached_dir in win_inst_dir() is not freed on unload. It should be cleaner to free it somehow.

[thep]

It seems one comment I made before has been lost. So, let me repeat it here:

In brk_load_default_dict() [brk-common.c], this part:

   /* Then, fall back to default DICT_DIR macro */
    if (!dict_trie) {
#if defined (_WIN32) && !defined (__CYGWIN__)
        {
            /* ... */
        }
        if (!dict_trie)
            dict_trie = trie_new_from_file (DICT_DIR "/" DICT_NAME ".tri");
#else
        dict_trie = trie_new_from_file (DICT_DIR "/" DICT_NAME ".tri");
#endif
    }

can be rearranged like this:

#if defined (_WIN32) && !defined (__CYGWIN__)
    /* Try to find dict under the base dir used to install the DLL */
    if (!dict_trie) {
        /* ... */
    }
#endif

   /* Then, fall back to default DICT_DIR macro */
    if (!dict_trie) {
        dict_trie = trie_new_from_file (DICT_DIR "/" DICT_NAME ".tri");
    }

That is, the Windows-specific stuff becomes just another step in between.

[thep]

It's not quite good to refer to a file from different directory in Makefile.am. For example, this makes it more complicated to check whether a file is included in the release tarball prepared with make dist.

Instead, we may need to create another internal library libutils.la under the utils/ subdir. That is:

• In src/Makefile.am, add $(top_builddir)/src/utils/libutils.la to libthai_sublibs. Previously, the utils/ subdir just provided a plain header. Now it does provide some binary code.
• In src/utils/Makefile.am, replace the noinst_HEADERS target with this (with win-utils.h split out of priv-utils.h):

noinst_LTLIBRARIES = libutils.la

libutils_la_SOURCES = \
        priv-utils.h \
        win-utils.h \
        win-utils.c \
        $(NULL)

Some source files that previously included priv-utils.h for the Windows stuff may need to include win-utils.h instead, of course.

[thep]

Looks good. Merged. Thank you very much for your work!

[bact]

Thanks for the reviews too.