We managed not to propagate configure-time CFLAGS to the makefile.
Fix that, and while there, make sure that gcc/clang-specific compiler
flags (for compilation mode etc.) are only set if Autoconf detects
a GCC-compatible compiler.

* configure.ac (CMAINTAINERFLAGS, CXXMAINTAINERFLAGS): Do not
   initialize from environment.
* Makefile.in (CFLAGS): Add substituted CUSERFLAGS to the definition.
  (CXXFLAGS): Likewise, with CXXUSERFLAGS.
* build/ac-macros/compiler.m4
  (SVN_CC_MODE_SETUP, SVN_CXX_MODE_SETUP): Add specific mode flags
   only if the corresponding $GCC or $GXX variable is set.
   Substitute CUSERFLAGS and CXXUSERFLAGS.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1518932 13f79535-47bb-0310-9956-ffa450edef68