Merge bitcoin-core/secp256k1#1160: Makefile: add `-I$(top_srcdir)/{in…

…clude,src}` to `CPPFLAGS` for precomputed

e862c4a Makefile: add -I$(top_srcdir)/src to CPPFLAGS for precomputed (Matt Whitlock)

Pull request description:

  When performing an out-of-source-tree build, regenerating the source files for the precomputed ecmult tables places them outside the source tree. Then, when they are to be compiled, they cannot find the headers they need because the source tree is absent from their include search path. This appears to have been an oversight, as the relevant `-I` options are present in `libsecp256k1_la_CPPFLAGS` but were missing from `libsecp256k1_precomputed_la_CPPFLAGS`. This PR adds them.

ACKs for top commit:
  sipa:
    utACK e862c4a
  real-or-random:
    ACK e862c4a

Tree-SHA512: f58b8670b2798f2ca4bd6e9fd83218afcd14cf1b796cd18fb40e7b8a148dcdfabe5f0beae81bc6b82727c97a507431e6a7c72d756587e047daf1ea81242cccf9

Makefile.am

@@ -73,7 +73,9 @@ noinst_HEADERS += examples/random.h
 PRECOMPUTED_LIB = libsecp256k1_precomputed.la
 noinst_LTLIBRARIES = $(PRECOMPUTED_LIB)
 libsecp256k1_precomputed_la_SOURCES =  src/precomputed_ecmult.c src/precomputed_ecmult_gen.c
-libsecp256k1_precomputed_la_CPPFLAGS = $(SECP_INCLUDES) $(SECP_CONFIG_DEFINES)
+# We need `-I$(top_srcdir)/src` in VPATH builds if libsecp256k1_precomputed_la_SOURCES have been recreated in the build tree.
+# This helps users and packagers who insist on recreating the precomputed files (e.g., Gentoo).
+libsecp256k1_precomputed_la_CPPFLAGS = -I$(top_srcdir)/src $(SECP_INCLUDES) $(SECP_CONFIG_DEFINES)
 
 if USE_EXTERNAL_ASM
 COMMON_LIB = libsecp256k1_common.la