New autoconf test for iconv

On a Solaris machine I have access to libc contains the symbol
"iconv" but, when compiling with gcc and including iconv.h we get
iconv.h from GNU libiconv. This header file define (among other
things) "iconv" to "libiconv" and so on.

In order to link with GNU libiconv we need -liconv. Currently we
test if the symbol "iconv" is in libc (which is true), then we get
a undefined reference error because we don't have libiconv_open.

The solution this patch implements is to compile and link a
small test program, instead of just checking if the libraries
(libc and libiconv) contains the symbol "iconv".

Signed-off-by: Fredrik Kuivinen <frekui@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

configure.ac

@@ -114,13 +114,32 @@ AC_CHECK_LIB([expat], [XML_ParserCreate],
 [NO_EXPAT=YesPlease])
 AC_SUBST(NO_EXPAT)
 #
-# Define NEEDS_LIBICONV if linking with libc is not enough (Darwin).
+# Define NEEDS_LIBICONV if linking with libc is not enough (Darwin and
+# some Solaris installations).
 # Define NO_ICONV if neither libc nor libiconv support iconv.
-AC_CHECK_LIB([c], [iconv],
-	[NEEDS_LIBICONV=],
-	AC_CHECK_LIB([iconv], [iconv],
-		[NEEDS_LIBICONV=YesPlease],
-		[NO_ICONV=YesPlease]))
+AC_DEFUN([ICONVTEST_SRC], [
+#include <iconv.h>
+
+int main(void)
+{
+	iconv_open("", "");
+	return 0;
+}
+])
+AC_MSG_CHECKING([for iconv in -lc])
+AC_LINK_IFELSE(ICONVTEST_SRC,
+	[AC_MSG_RESULT([yes])
+	NEEDS_LIBICONV=],
+	[AC_MSG_RESULT([no])
+	old_LIBS="$LIBS"
+	LIBS="$LIBS -liconv"
+	AC_MSG_CHECKING([for iconv in -liconv])
+	AC_LINK_IFELSE(ICONVTEST_SRC,
+		[AC_MSG_RESULT([yes])
+		NEEDS_LIBICONV=YesPlease],
+		[AC_MSG_RESULT([no])
+		NO_ICONV=YesPlease])
+	LIBS="$old_LIBS"])
 AC_SUBST(NEEDS_LIBICONV)
 AC_SUBST(NO_ICONV)
 test -n "$NEEDS_LIBICONV" && LIBS="$LIBS -liconv"