build: Fix configure.ac indentation

Use the same style as the newly added code, which should make the code
more readable.

configure.ac

@@ -6,10 +6,18 @@ AC_CONFIG_SRCDIR([src/sha2.c])
 AC_CONFIG_AUX_DIR([build-aux])
 AC_CONFIG_MACRO_DIR([m4])
 
-AM_INIT_AUTOMAKE([1.8 -Wall foreign nostdinc no-dist-gzip dist-xz])
-
-m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])],
-                            [AC_SUBST([AM_DEFAULT_VERBOSITY], [1])])
+AM_INIT_AUTOMAKE(
+  [1.8]
+  [-Wall]
+  [foreign]
+  [nostdinc]
+  [no-dist-gzip dist-xz]
+)
+
+m4_ifdef([AM_SILENT_RULES],
+  [AM_SILENT_RULES([yes])],
+  [AC_SUBST([AM_DEFAULT_VERBOSITY], [1])]
+)
 
 LIBMD_ABI_MAJOR=0
 LIBMD_ABI_MINOR=0
@@ -100,12 +108,12 @@ AC_TYPE_SSIZE_T
 # Checks for library functions.
 
 AC_CONFIG_FILES([
-	Makefile
-	include/Makefile
-	man/Makefile
-	src/Makefile
-	src/libmd.pc
-	test/Makefile
+  Makefile
+  include/Makefile
+  man/Makefile
+  src/Makefile
+  src/libmd.pc
+  test/Makefile
 ])
 AC_CONFIG_HEADERS([config.h])
 AC_OUTPUT