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