Windows MSVC build: more compat-layer fixes

Round 2 of c67b631 cleanup.  After the dirent + utime fixes, the
MSVC link surface still had:

- LNK2005 'fopen already defined': dropped g_fopen so we no longer
  override the SDK's fopen.  UTF-8 paths still work on Windows 10
  with the active-codepage manifest; non-Unicode codepages will see
  ANSI translation.  This is good enough for the public release; a
  full UTF-8 fopen wrapper can be added later if needed.

- LNK2019 'unresolved S_ISDIR / S_ISREG': MSVC's <sys/stat.h> defines
  _S_IFDIR / _S_IFREG but not the POSIX S_IS* macros.  Add them in
  the unistd.h shim (which main.c already pulls).

- LNK1181 'cannot open input file m.lib': test_merkle and test_rans
  linked libm unconditionally.  Math is in the default CRT on MSVC;
  link 'm' only on non-Windows.

- 'unistd.h' not found in test_blockstore.c: it actually only needs
  getpid().  Use <process.h> + #define getpid _getpid on MSVC, keep
  <unistd.h> elsewhere.

cli/CMakeLists.txt

@@ -29,7 +29,7 @@ if(WIN32)
         NO_OLDNAMES
         g_err g_errx g_warn g_warnx g_vwarn g_vwarnx g_verr g_verrx
         g_getprogname g_setlinebuf g_fnmatch
-        g_compat__utf8_console g_compat__wpath g_fopen
+        g_compat__utf8_console g_compat__wpath
         g_access g_unlink g_chdir g_mkdir g_chmod g_utime
         g_opendir
     )

cli/src/compat/include/msvc/unistd.h

@@ -24,6 +24,14 @@
 #define PATH_MAX 260
 #endif
 
+#include <sys/stat.h>
+#ifndef S_ISDIR
+#define S_ISDIR(m) (((m) & _S_IFMT) == _S_IFDIR)
+#endif
+#ifndef S_ISREG
+#define S_ISREG(m) (((m) & _S_IFMT) == _S_IFREG)
+#endif
+
 /* Provided by compat_win32.c (UTF-8-aware via wide-char APIs) */
 int access(const char *path, int mode);
 int unlink(const char *path);

tests/CMakeLists.txt

@@ -52,7 +52,11 @@ target_compile_features(test_cdc PRIVATE c_std_99)
 add_test(NAME cdc COMMAND test_cdc)
 
 add_executable(test_merkle src/test_merkle.c)
-target_link_libraries(test_merkle PRIVATE uc2 m)
+if(WIN32)
+    target_link_libraries(test_merkle PRIVATE uc2)
+else()
+    target_link_libraries(test_merkle PRIVATE uc2 m)
+endif()
 target_include_directories(test_merkle PRIVATE "${PROJECT_BINARY_DIR}/lib")
 target_compile_features(test_merkle PRIVATE c_std_99)
 add_test(NAME merkle COMMAND test_merkle)
@@ -76,7 +80,11 @@ target_compile_features(test_delta PRIVATE c_std_99)
 add_test(NAME delta COMMAND test_delta)
 
 add_executable(test_rans src/test_rans.c)
-target_link_libraries(test_rans PRIVATE uc2 m)
+if(WIN32)
+    target_link_libraries(test_rans PRIVATE uc2)
+else()
+    target_link_libraries(test_rans PRIVATE uc2 m)
+endif()
 target_include_directories(test_rans PRIVATE "${PROJECT_BINARY_DIR}/lib")
 target_compile_features(test_rans PRIVATE c_std_99)
 add_test(NAME rans COMMAND test_rans)

tests/src/test_blockstore.c

@@ -4,7 +4,12 @@
 #include <stdlib.h>
 #include <string.h>
 #include <assert.h>
+#ifdef _MSC_VER
+#include <process.h>
+#define getpid _getpid
+#else
 #include <unistd.h>
+#endif
 #include <uc2/uc2_blockstore.h>
 #include <uc2/uc2_merkle.h>