add support for compiling fully static LAMMPS executable with libcurl

cmake/Modules/Packages/EXTRA-COMMAND.cmake

@@ -3,8 +3,16 @@
 find_package(CURL QUIET)
 option(WITH_CURL "Enable libcurl support" ${CURL_FOUND})
 if(WITH_CURL)
-  find_package(CURL REQUIRED)
   target_compile_definitions(lammps PRIVATE -DLAMMPS_CURL)
-  target_link_libraries(lammps PRIVATE CURL::libcurl)
+
+  # need to use pkgconfig for fully static bins to find custom static libs
+  if (CMAKE_SYSTEM_NAME STREQUAL "LinuxMUSL")
+    include(FindPkgConfig)
+    pkg_check_modules(CURL IMPORTED_TARGET libcurl libssl libcrypto)
+    target_link_libraries(lammps PUBLIC PkgConfig::CURL)
+  else()
+    find_package(CURL REQUIRED)
+    target_link_libraries(lammps PRIVATE CURL::libcurl)
+  endif()
 endif()