[Autotools] Unicode's CFLAGS enforce -D_FORTIFY_SOURCE=2, -D_REENTRANT=1, causing faulty Clang builds
https://bugs.webkit.org/show_bug.cgi?id=119685

Reviewed by Gustavo Noronha Silva.

icu-config includes '-D_FORTIFY_SOURCE=2 -D_REENTRANT=1' when printing out C preprocessor flags that are used
as the C compiler flags to avoid other unwanted compiler options. This causes problems when building optimized
builds with Clang because of a bug in that compiler:
http://llvm.org/bugs/show_bug.cgi?id=16821

To avoid that, the C preprocessor search flags, as printed by `icu-config --cppflags-searchpath` are now used
the Unicode dependency's C compiler flags, avoiding unconditionally specifying the two macros.

To adjust for that, the `-pthread` flag is added to the global CFLAGS and CXXFLAGS variables, ensuring
the _REENTRANT define is set to 1 and declaring the flag globally instead of relying on Glib dependency's
C compiler flags to do so for us. -D_FORTIFY_SOURCE=2 is only added to the CFLAGS and CXXFLAGS variables if
the compiler in use is gcc or g++, preventing the Clang builds to malfunction.

* Source/autotools/FindDependencies.m4:
* Source/autotools/SetupCompilerFlags.m4:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154008 268f45cc-cd09-0410-ab3c-d52691b4dbfc