meson: Basic cygwin support

There likely are further issues, but as evidenced by the CI task proposed by
Justin in the referenced thread, this suffices to build and run basic tests in
cygwin (some fixes for the test infrastructure are needed, but that's
independent of the meson aspect).

Author: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20221021034040.GT16921@telsasoft.com

meson.build

@@ -211,6 +211,10 @@ if host_system == 'aix'
 
 elif host_system == 'cygwin'
   cppflags += '-D_GNU_SOURCE'
+  dlsuffix = '.dll'
+  mod_link_args_fmt = ['@0@']
+  mod_link_with_name = 'lib@0@.exe.a'
+  mod_link_with_dir = 'libdir'
 
 elif host_system == 'darwin'
   dlsuffix = '.dylib'
@@ -2310,8 +2314,8 @@ gnugetopt_dep = cc.find_library('gnugetopt', required: false)
 #   (i.e., allow '-' as a flag character), so use our version on those platforms
 # - We want to use system's getopt_long() only if the system provides struct
 #   option
-always_replace_getopt = host_system in ['windows', 'openbsd', 'solaris']
-always_replace_getopt_long = host_system == 'windows' or not cdata.has('HAVE_STRUCT_OPTION')
+always_replace_getopt = host_system in ['windows', 'cygwin', 'openbsd', 'solaris']
+always_replace_getopt_long = host_system in ['windows', 'cygwin'] or not cdata.has('HAVE_STRUCT_OPTION')
 
 # Required on BSDs
 execinfo_dep = cc.find_library('execinfo', required: false)

src/port/meson.build

@@ -40,6 +40,10 @@ if host_system == 'windows'
     'win32setlocale.c',
     'win32stat.c',
   )
+elif host_system == 'cygwin'
+  pgport_sources += files(
+    'dirmod.c',
+  )
 endif
 
 if cc.get_id() == 'msvc'

src/test/regress/meson.build

@@ -12,6 +12,8 @@ regress_sources = pg_regress_c + files(
 host_tuple_cc = cc.get_id()
 if host_system == 'windows' and host_tuple_cc == 'gcc'
   host_tuple_cc = 'mingw'
+elif host_system == 'cygwin' and host_tuple_cc == 'gcc'
+  host_tuple_cc = 'cygwin'
 endif
 host_tuple = '@0@-@1@-@2@'.format(host_cpu, host_system, host_tuple_cc)