RFC: Build system changes

The attached patch makes several changes to how we discover compilers
and set flags on *nix:

  * Search for clang as well as the default gcc/cc, and prefer clang(++)
    over gcc/g++.
  * Set standards-compliance mode (C90/C++11) even without maintainer-mode.
  * Add -pipe to C(XX)FLAGS if the compiler supports it. This speeds up
    compilation a bit in my tests.

I tested this on Mac OS (with clang) and Linux (without clang) but don't
know how it affects bindings.

Please review.

-- Brane

{{{
* build/ac-macros/compiler.m4: New file.
  (SVN_PROG_CC, SVN_CFLAGS_ADD_IFELSE,
   SVN_PROG_CXX, SVN_CXXFLAGS_ADD_IFELSE): New.
* aclocal.m4: Include build/ac-macros/compiler.m4.

* configure.ac:
  - Use SVN_PROG_CC instead of AC_PROG_CC
  - Use SVN_PROG_CXX instead of AC_PROG_CXX
  - Use SVN_CFLAGS_ADD_IFELSE and SVN_CXXFLAGS_ADD_IFELSE
    to add mainter-mode flags
  - Do not remove -ansi for clang, since it's not set any more.
}}}

-- 
Branko Čibej
Director of Subversion | WANdisco | www.wandisco.com