fix configure.ac

Into the darwin OS detection the AC_EGREP_CPP macro is used.
This causes an bugged configure file issued and this one
prevents Linux platform to generate shared-library.

Following this link:
https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/AC_005fACT_005fIFELSE-vs-AC_005fTRY_005fACT.html

Using AC_EGREP_CPP is a bad way to test a C preprocessor.
This patch changes this call by AC_COMPILE_IFELSE + AC_LANG_PROGRAM calls:
- this is a better way to process
- this fix the linux shared build

Change-Id: I150666f2bc45a48d021110025f2d486a9750d333
Tuleap: #699

configure.ac

@@ -50,14 +50,14 @@ case "${host_os}" in
     ;;
   darwin*)
     SYS=darwin
-
-    AC_EGREP_CPP(yes,
-            [#import <TargetConditionals.h>
-             #if TARGET_OS_IPHONE
-             yes
-             #endif],
-             [HAVE_IOS="1"],
-             [HAVE_OSX="1"])
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+        [[#import <TargetConditionals.h>
+          #if TARGET_OS_IPHONE == 0
+          #error this is not an IPHONE
+          #endif
+        ]])],
+        [HAVE_IOS="1"],
+        [HAVE_OSX="1"])
     ;;
   mingw32*)
     SYS=mingw32