cmake: Build secp256k1 only when required

Author: hebasto

src/CMakeLists.txt

@@ -73,6 +73,9 @@ if(DEFINED ENV{CFLAGS})
 endif()
 set(CMAKE_EXPORT_COMPILE_COMMANDS OFF)
 add_subdirectory(secp256k1)
+set_target_properties(secp256k1 PROPERTIES
+  EXCLUDE_FROM_ALL TRUE
+)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 string(APPEND CMAKE_C_COMPILE_OBJECT " ${APPEND_CPPFLAGS} ${APPEND_CFLAGS}")