This method ensure that all users of glog get automatically linked to
the DbgHelp library without needing to set compiler flags.

Necessary when building with BUILD_SHARED_LIBS=1.

Remove files generated by autotools

closes #165 

for #165. I think not many projects have them in their repo.

Backport internal changes

Don't rely on an internal-linkage extern "C" function having an
unmangled name. This isn't required by the ABI, and in fact is not
valid for a conforming compiler(!). Instead, allow symbolization to
produce either a mangled or an unmangled name here.

CHECK_NOTNULL works with smart pointers when compiled in C++11.

Add __declspec(noreturn) on Win

Currently cl.exe doesn't know LOG(FATAL) exits the program. Set
__declspec(noreturn).

DCHECK_ALWAYS_ON to make D* enabled under NDEBUG

The macro NDEBUG could be automatically defined for release build on
some build environments (e.g. MSVC).  If we use NDEBUG as a key to
distinguish using DCHECK as CHECK (I call this DCHECK is enabled) or
not, we cannot make DCHECK enabled for release build on such
environments.

Considering people use a program with glog for presubmit testing or
dogfooding, they should need to do release build with DCHECK enabled.

cmake: allow to disable building unit tests

This commit introduces a CMake `BUILD_TESTING` boolean option that
allows to disable unit tests. This is especially useful in superbuilds
where typically only the main library is built. By default, this option
is enabled.

try to avoid the error "conflicting declaration 'typedef DWORD pthread_t'" etc. in MinGW

Set sinks_ to NULL after deletion in LogDestination::DeleteLogDestinations

Fix autotools build.

Added $<BUILD_INTERFACE> to gflags include directory

cmake: enable position independent code (PIC)

cmake: fixed clang warning propagation to gcc

pthread_t'" etc.