Tracking issue for manual (non-autotools) builds

[real-or-random]

The current method to build a static library manually (using default options) is:

gcc -O2 -c src/secp256k1.c src/precomputed_*.c 
ar rcs libsecp256k1.a secp256k1.o precomputed_*.o

(clang also works)

The goal is to get rid of most of the command line arguments, at least on GCC and clang:

• -DSECP256K1_BUILD: Define SECP256K1_BUILD in secp256k1.c directly. #928
• -I changed include statements without prefix 'include/'  #925
• -D ECMULT_GEN_PREC_BITS=4 -D ECMULT_WINDOW_SIZE=15: See discussion in Clean up configuration in gen_context  #918. We could even print the config using #pragma message)
• -D DUSE_EXTERNAL_ASM: Tracking issue for manual (non-autotools) builds #929 (comment)
• -Wno-unused-function: We could add a #pragma or better add __attribute__ ((unsused)) to the relevant functions.
• -Wall -Wextra: We could set this via #pragmas but maybe we should leave this to the user.
• -O2: I think we should really leave this to the user.
• Add section to README
• Add CI jobs to test these builds

[real-or-random]

-Wno-unused-function: We could add a #pragma or better add attribute ((unsused)) to the relevant functions.

There are no unused functions except for ones that are unneeded because of test harness or module configuration options.

Sure? I think at least the multimultiplication functions and the scratch space functions are currently unused.

[real-or-random]

Now that #1042 has been merged, we need to build multiple modules:

gcc -c src/secp256k1.c src/precomputed_*.c -DECMULT_GEN_PREC_BITS=4 -DECMULT_WINDOW_SIZE=15
ar rcs libsecp256k1.a secp256k1.o precomputed_*.o