build: don't duplicate substitution of make variables

Thanks to our 'GIT_CONF_SUBST' layer in configure.ac, a make variable 'VAR'
can be defined to a value 'VAL' at ./configure runtime in our build system
simply by using "GIT_CONF_SUBST([VAR], [VAL])" in configure.ac, rather than
having both to call "AC_SUBST([VAR], [VAL])" in configure.ac and adding the
'VAR = @var@' definition in config.mak.in.  Less duplication, less margin
for error, less possibility of confusion.

While at it, fix some formatting issues in configure.ac that unnecessarily
obscured the code flow.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: slattarini

config.mak.in

@@ -5,12 +5,10 @@ CC = @CC@
 CFLAGS = @CFLAGS@
 CPPFLAGS = @CPPFLAGS@
 LDFLAGS = @LDFLAGS@
-CC_LD_DYNPATH = @CC_LD_DYNPATH@
 AR = @AR@
 TAR = @TAR@
 DIFF = @DIFF@
 #INSTALL = @INSTALL@		# needs install-sh or install.sh in sources
-TCLTK_PATH = @TCLTK_PATH@
 
 prefix = @prefix@
 exec_prefix = @exec_prefix@
@@ -27,50 +25,3 @@ VPATH = @srcdir@
 
 export exec_prefix mandir
 export srcdir VPATH
-
-NEEDS_SSL_WITH_CRYPTO=@NEEDS_SSL_WITH_CRYPTO@
-NO_OPENSSL=@NO_OPENSSL@
-NO_CURL=@NO_CURL@
-NO_EXPAT=@NO_EXPAT@
-NO_LIBGEN_H=@NO_LIBGEN_H@
-HAVE_PATHS_H=@HAVE_PATHS_H@
-HAVE_LIBCHARSET_H=@HAVE_LIBCHARSET_H@
-NO_GETTEXT=@NO_GETTEXT@
-LIBC_CONTAINS_LIBINTL=@LIBC_CONTAINS_LIBINTL@
-NEEDS_LIBICONV=@NEEDS_LIBICONV@
-NEEDS_SOCKET=@NEEDS_SOCKET@
-NEEDS_RESOLV=@NEEDS_RESOLV@
-NEEDS_LIBGEN=@NEEDS_LIBGEN@
-NO_SYS_SELECT_H=@NO_SYS_SELECT_H@
-NO_D_INO_IN_DIRENT=@NO_D_INO_IN_DIRENT@
-NO_D_TYPE_IN_DIRENT=@NO_D_TYPE_IN_DIRENT@
-NO_SOCKADDR_STORAGE=@NO_SOCKADDR_STORAGE@
-NO_IPV6=@NO_IPV6@
-NO_HSTRERROR=@NO_HSTRERROR@
-NO_STRCASESTR=@NO_STRCASESTR@
-NO_STRTOK_R=@NO_STRTOK_R@
-NO_FNMATCH=@NO_FNMATCH@
-NO_FNMATCH_CASEFOLD=@NO_FNMATCH_CASEFOLD@
-NO_MEMMEM=@NO_MEMMEM@
-NO_STRLCPY=@NO_STRLCPY@
-NO_UINTMAX_T=@NO_UINTMAX_T@
-NO_STRTOUMAX=@NO_STRTOUMAX@
-NO_SETENV=@NO_SETENV@
-NO_UNSETENV=@NO_UNSETENV@
-NO_MKDTEMP=@NO_MKDTEMP@
-NO_MKSTEMPS=@NO_MKSTEMPS@
-NO_INET_NTOP=@NO_INET_NTOP@
-NO_INET_PTON=@NO_INET_PTON@
-NO_ICONV=@NO_ICONV@
-OLD_ICONV=@OLD_ICONV@
-NO_REGEX=@NO_REGEX@
-USE_LIBPCRE=@USE_LIBPCRE@
-NO_DEFLATE_BOUND=@NO_DEFLATE_BOUND@
-INLINE=@INLINE@
-SOCKLEN_T=@SOCKLEN_T@
-FREAD_READS_DIRECTORIES=@FREAD_READS_DIRECTORIES@
-SNPRINTF_RETURNS_BOGUS=@SNPRINTF_RETURNS_BOGUS@
-NO_PTHREADS=@NO_PTHREADS@
-PTHREAD_CFLAGS=@PTHREAD_CFLAGS@
-PTHREAD_LIBS=@PTHREAD_LIBS@
-CHARSET_LIB=@CHARSET_LIB@