Simplify autotools build

[mgreter]

• Generate sass_version.h
• Use $(EXEEXT) for executables
• No need for -lm and C++ library.
• They should be provided by libtool.

Work done by @saper !

[coveralls]

Coverage remained the same at 80.37% when pulling 3ffa142 on mgreter:dlopen into cfce077 on sass:master.

[saper]

Can we also have AM_PROG_AR on mingw32? Otherwise autoreconf complains out:

/usr/local/share/automake-1.15/am/ltlibrary.am: warning: 'libsass.la': linking libtool libraries using a non-POSIX
/usr/local/share/automake-1.15/am/ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
Makefile.am:28:   while processing Libtool library 'libsass.la'

[saper]

Looks like _AC_PROG_CC_G autoconf test sets -O2 -g as the default compiler flags on gcc-likes (including clang).

[drewwells]

just as a sanity check, b/c this has been changing a lot lately. Here's the params I pass to build libsass

-lsass -lsdc++ -lm -ldl -std=c++11

lm is removed by this PR and I think ldl was removed in another. Which of these params should I be passing now?

[saper]

Can you try running autoreconf and then ./configure ? libtool should find them