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

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 commit adds them.

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