autotools: Rename build-aux to autotools-aux

This change improves separation from CMake build directories, which
typically use the "build" prefix.

Additionally, corresponding `.gitignore` entries have been refactored.

.gitignore

@@ -45,20 +45,9 @@ coverage.*.html
 *.gcno
 *.gcov
 
-build-aux/ar-lib
-build-aux/config.guess
-build-aux/config.sub
-build-aux/depcomp
-build-aux/install-sh
-build-aux/ltmain.sh
-build-aux/m4/libtool.m4
-build-aux/m4/lt~obsolete.m4
-build-aux/m4/ltoptions.m4
-build-aux/m4/ltsugar.m4
-build-aux/m4/ltversion.m4
-build-aux/missing
-build-aux/compile
-build-aux/test-driver
+/autotools-aux/
+!/autotools-aux/m4/bitcoin_secp.m4
+
 libsecp256k1.pc
 
 ### CMake

Makefile.am

@@ -1,4 +1,4 @@
-ACLOCAL_AMFLAGS = -I build-aux/m4
+ACLOCAL_AMFLAGS = -I autotools-aux/m4
 
 # AM_CFLAGS will be automatically prepended to CFLAGS by Automake when compiling some foo
 # which does not have an explicit foo_CFLAGS variable set.

configure.ac

@@ -19,8 +19,8 @@ define(_LIB_VERSION_AGE, 0)
 
 AC_INIT([libsecp256k1],m4_join([.], _PKG_VERSION_MAJOR, _PKG_VERSION_MINOR, _PKG_VERSION_PATCH)m4_if(_PKG_VERSION_IS_RELEASE, [true], [], [-dev]),[https://github.com/bitcoin-core/secp256k1/issues],[libsecp256k1],[https://github.com/bitcoin-core/secp256k1])
 
-AC_CONFIG_AUX_DIR([build-aux])
-AC_CONFIG_MACRO_DIR([build-aux/m4])
+AC_CONFIG_AUX_DIR([autotools-aux])
+AC_CONFIG_MACRO_DIR([autotools-aux/m4])
 AC_CANONICAL_HOST
 
 # Require Automake 1.11.2 for AM_PROG_AR

src/CMakeLists.txt

@@ -95,7 +95,7 @@ set_target_properties(secp256k1_objs PROPERTIES
 )
 
 # This emulates Libtool to make sure Libtool and CMake agree on the ABI version,
-# see below "Calculate the version variables" in build-aux/ltmain.sh.
+# see below "Calculate the version variables" in autotools-aux/ltmain.sh.
 math(EXPR ${PROJECT_NAME}_soversion "${${PROJECT_NAME}_LIB_VERSION_CURRENT} - ${${PROJECT_NAME}_LIB_VERSION_AGE}")
 set_target_properties(secp256k1 PROPERTIES
   SOVERSION ${${PROJECT_NAME}_soversion}