Add a secp256k1 makefile and building instructions (#24)

.gitmodules

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+[submodule "secp256k1/secp256k1-zkp"]
+	path = secp256k1/secp256k1-zkp
+	url = https://github.com/ElementsProject/secp256k1-zkp.git

secp256k1/Makefile

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+CROSS_DLL ?= 0
+TARGET = libsecp256k1
+ifeq ($(CROSS_DLL),1)
+PLATFORM = windows
+ARCH = amd64
+else ifeq ($(OS),Windows_NT)
+PLATFORM = windows
+ARCH = amd64
+else
+PLATFORM = $(shell uname -s | tr A-Z a-z)
+ARCH = $(shell uname -m)
+endif
+
+# Paths
+LIB_DIR = secp256k1-zkp
+BUILD_DIR = build
+
+# Tools
+ifeq ($(PLATFORM),windows)
+TOOLCHAIN_PREFIX ?= x86_64-w64-mingw32-
+else
+TOOLCHAIN_PREFIX ?= 
+endif
+
+CC := $(TOOLCHAIN_PREFIX)gcc
+ifeq ($(OS),Windows_NT)
+MKDIR_P = mkdir
+RM_R = rmdir /s /q
+else
+MKDIR_P = mkdir -p
+RM_R = rm -r
+endif
+
+# C sources
+C_SOURCES = $(addprefix $(LIB_DIR)/src/,\
+	secp256k1.c \
+	)
+
+# C includes
+C_INCLUDES =  \
+	$(LIB_DIR) \
+	$(LIB_DIR)/src \
+	config
+
+# C defines
+C_DEFS =  \
+	HAVE_CONFIG_H
+
+ifeq ($(PLATFORM),windows)
+C_DEFS += _WIN32
+endif
+
+OBJS := $(addprefix $(BUILD_DIR)/,$(notdir $(C_SOURCES:.c=.o)))
+vpath %.c $(sort $(dir $(C_SOURCES)))
+
+DEPS := $(OBJS:.o=.d)
+
+ifeq ($(PLATFORM),windows)
+CFLAGS = -O2 -std=c99 -MMD -MP -Werror -Wno-unused-function \
+	$(addprefix -I,$(C_INCLUDES)) $(addprefix -D,$(C_DEFS))
+else
+CFLAGS = -fPIC -O2 -Werror -Wno-unused-function \
+	$(addprefix -I,$(C_INCLUDES)) $(addprefix -D,$(C_DEFS))
+endif
+
+ifeq ($(PLATFORM),windows)
+LDFLAGS = -shared -s \
+	-Wl,--subsystem,windows,--out-implib,$(BUILD_DIR)/$(TARGET).a
+EXT = .dll
+else ifeq ($(PLATFORM),darwin)
+LDFLAGS = -dynamiclib
+EXT = .dylib
+else
+LDFLAGS = -shared
+EXT = .so
+endif
+
+$(BUILD_DIR)/$(TARGET)_$(PLATFORM)_$(ARCH)$(EXT): $(OBJS) Makefile
+	$(CC) $(OBJS) -o $@ $(LDFLAGS)
+
+$(BUILD_DIR)/%.o: %.c Makefile
+	$(MKDIR_P) "$(dir $@)"
+	$(CC) $(CFLAGS) -c $< -o $@
+
+.PHONY: clean
+
+clean:
+	$(RM_R) "$(BUILD_DIR)"
+
+-include $(DEPS)

secp256k1/README.md

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+# Building secp256k1 for embit
+
+If you don't want to use prebuilt binary packaged with `embit` you can build it yourself.
+
+We are using **libsecp256k1** fork -
+[**secp256k1-zkp**](https://github.com/ElementsProject/secp256k1-zkp).
+
+# Building the library under target platform
+
+To build the library run:
+
+```sh
+make
+```
+
+To clean build directory use:
+
+```shell
+make clean
+```
+
+# Cross-compiling Windows DLL
+
+## Toolchain install
+
+### Linux
+
+In the console type:
+
+```shell
+sudo apt-get install gcc-mingw-w64-x86-64 g++-mingw-w64-x86-64 wine64
+```
+
+### Mac
+
+Assuming that [Homebrew](https://brew.sh/) package manager is installed, in the console type:
+
+```shell
+brew install mingw-w64
+brew install --cask xquartz
+brew install --cask wine-stable
+```
+
+### Windows
+
+Assuming that [Chocolatey](https://chocolatey.org/) package manager is installed, in the **Powershell** type:
+
+```shell
+choco install mingw make
+```
+
+## Building the library
+
+To build the Windows DLL and the companion library from other platforms run:
+
+```shell
+make CROSS_DLL=1
+```

secp256k1/config/libsecp256k1-config.h

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+#ifndef SECP256K1_CONFIG_H
+#define SECP256K1_CONFIG_H
+
+#undef USE_ASM_X86_64
+#undef USE_ECMULT_STATIC_PRECOMPUTATION
+#undef USE_ENDOMORPHISM
+#undef USE_EXTERNAL_ASM
+#undef USE_EXTERNAL_DEFAULT_CALLBACKS
+#undef USE_FIELD_10X26
+#undef USE_FIELD_5X52
+#undef USE_FIELD_INV_BUILTIN
+#undef USE_FIELD_INV_NUM
+#undef USE_NUM_GMP
+#undef USE_NUM_NONE
+#undef USE_SCALAR_4X64
+#undef USE_SCALAR_8X32
+#undef USE_SCALAR_INV_BUILTIN
+#undef USE_SCALAR_INV_NUM
+#undef ECMULT_WINDOW_SIZE
+
+#define ENABLE_MODULE_ECDH 1
+#define ENABLE_MODULE_RECOVERY 1
+#define ENABLE_MODULE_GENERATOR 1
+#define ENABLE_MODULE_RANGEPROOF 1
+#define ENABLE_MODULE_SURJECTIONPROOF 1
+#define ENABLE_MODULE_MUSIG 1
+#define ENABLE_MODULE_EXTRAKEYS 1
+#define ENABLE_MODULE_SCHNORRSIG 1
+
+
+#define USE_NUM_NONE 1
+#define USE_FIELD_INV_BUILTIN 1
+#define USE_SCALAR_INV_BUILTIN 1
+#define USE_FIELD_10X26 1
+#define USE_SCALAR_8X32 1
+
+#define ECMULT_GEN_PREC_BITS 4
+#define ECMULT_WINDOW_SIZE 4
+
+#define HAVE_STDINT_H 1
+#define HAVE_STDLIB_H 1
+#define HAVE_STRING_H 1
+
+#endif /* SECP256K1_CONFIG_H */

secp256k1/secp256k1-zkp

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+Subproject commit d9560e0af78d9059bba0c4845a310387abfa4e5e