Merge #20469: build: Avoid secp256k1.h include from system

e95aaef build: Avoid secp256k1.h include from system (Niklas Gögge)

Pull request description:

  While building i ran into an error because i had a version of `secp256k1.h` under `/usr/local/include` that was incompatible with the secp256k1 code in the repository. This caused a problem because `$(BOOST_CPPFLAGS)` contained `-I/usr/local/include` and the include paths are searched by the compiler in order from left to right, so in the end `$(BITCOIN_INCLUDES)` contained `-I/usr/local/include` before `-I$(srcdir)/secp256k1/include` which caused the compiler to find  `secp256k1.h` under `/usr/local/include`.

  Looking at git blame i am wondering how this has not happened to anyone else in several years: https://github.com/bitcoin/bitcoin/blame/cb89e18845852c005ba8ab94acc6544d7c876a42/src/Makefile.am#L25

  I am on macOS 10.15.

ACKs for top commit:
  laanwj:
    Code review ACK e95aaef
  hebasto:
    ACK e95aaef, tested on macOS 11 Big Sur by adding `#error` into `/usr/local/include/secp256k1.h`.

Tree-SHA512: 1f0b395725936c179ab60dee3582ec7b21e2f9c0f1895e160d84a487cf0db16d0c7aa47d05800e0aded31685b4362056cac9b9ecca1bb8c308a4c5a810e8dc1d

Author: laanwj

src/Makefile.am

@@ -20,9 +20,8 @@ else
 LIBUNIVALUE = $(UNIVALUE_LIBS)
 endif
 
-BITCOIN_INCLUDES=-I$(builddir) $(BDB_CPPFLAGS) $(BOOST_CPPFLAGS) $(LEVELDB_CPPFLAGS)
+BITCOIN_INCLUDES=-I$(builddir) -I$(srcdir)/secp256k1/include $(BDB_CPPFLAGS) $(BOOST_CPPFLAGS) $(LEVELDB_CPPFLAGS)
 
-BITCOIN_INCLUDES += -I$(srcdir)/secp256k1/include
 BITCOIN_INCLUDES += $(UNIVALUE_CFLAGS)
 
 LIBBITCOIN_SERVER=libbitcoin_server.a