build: create a macro to check for, and replace, possibly-inline functions.

mingw (both 32 and 64) provides a number of functions that have no C
linkage, but are only available as static inline. Define a macro that can
check for the function declaration but acts like AC_REPLACE_FUNC.

Use the new macro for asprintf/vasprintf (previously implemented in
configure.ac directly).
Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

configure.ac

@@ -562,20 +562,8 @@ AC_CHECK_FUNCS(fdatasync,,
 ])
 
 dnl mingw64 implements those as static inline, not functions with C linkage
-AC_LINK_IFELSE([
-    AC_LANG_PROGRAM([#include <stdio.h>], [
-        char *c;
-        if (asprintf(&c, "%s %d", "string", 1) == -1)
-            c = NULL;
-    ])],[AC_DEFINE([HAVE_ASPRINTF],[1],[Define to 1 if you have asprintf function])],[AC_LIBOBJ([asprintf])])
-AC_LINK_IFELSE([
-    AC_LANG_PROGRAM([#include <stdio.h>
-                     #include <stdarg.h>], [
-        char *c;
-        va_list ap;
-        if (vasprintf(&c, "%s %d", ap) == -1)
-            c = NULL;
-    ])],[AC_DEFINE([HAVE_VASPRINTF],[1],[Define to 1 if you have asprintf function])],[AC_LIBOBJ([vasprintf])])
+VLC_REPLACE_DECL([asprintf], [#include <stdio.h>])
+VLC_REPLACE_DECL([vasprintf], [#include <stdio.h>])
 
 dnl C11 static_assert()
 AC_MSG_CHECKING([for static_assert in assert.h])

m4/vlc.m4

@@ -96,3 +96,23 @@ AC_DEFUN([VLC_LIBRARY_SUFFIX], [
   AC_MSG_RESULT(${LIBEXT})
   AC_DEFINE_UNQUOTED(LIBEXT, "${LIBEXT}", [Dynamic object extension])
 ])
+
+dnl ===========================================================================
+dnl  Custom macros for checking functions with inline fallback, for mingw32/64
+
+dnl VLC_REPLACE_DECL([funcname], [include])
+AC_DEFUN([VLC_REPLACE_DECL], [
+  AS_VAR_PUSHDEF([CACHEVAR], [vlc_cv_replace_decl] AS_TR_SH([$1]))
+  AC_CACHE_VAL(CACHEVAR, [
+    AC_CHECK_DECL(
+      [$1],
+      AS_VAR_SET(CACHEVAR, [yes]),
+      AS_VAR_SET(CACHEVAR, [no]),
+      [$2]
+    )
+  ])
+  AS_IF([test x"AS_VAR_GET(CACHEVAR)" = xyes],
+    [AC_DEFINE(AS_TR_CPP([HAVE_$1]), [1], [Define to 1 if you have $1 function])],
+    [AC_LIBOBJ([$1])])
+  AS_VAR_POPDEF([CACHEVAR])
+])