Fix incorrect CMAKE_SOURCE_DIR usage

When using solidity as a third-party library (include it into our
project using FetchContent), we encountered a strange compilation error.
For some reason, cmake considers the root directory of the project as
the root directory of the dependency (solidity). This is because
solidity is incorrectly using CMAKE_SOURCE_DIR variable, which should be
PROJECT_SOURCE_DIR (The former one refers to the top-level source
directory that contains a CMakeLists.txt, while the latter refers to the
source directory of the most recent project() command)

I've created a repo for demonstration (https://github.com/junaire/test-solidity-fetch-content)

Signed-off-by: Jun Zhang <jun@junz.org>

cmake/EthCompilerSettings.cmake

@@ -28,7 +28,7 @@ if(PEDANTIC)
 endif()
 
 # Prevent the path of the source directory from ending up in the binary via __FILE__ macros.
-eth_add_cxx_compiler_flag_if_supported("-fmacro-prefix-map=${CMAKE_SOURCE_DIR}=/solidity")
+eth_add_cxx_compiler_flag_if_supported("-fmacro-prefix-map=${PROJECT_SOURCE_DIR}=/solidity")
 
 # -Wpessimizing-move warns when a call to std::move would prevent copy elision
 # if the argument was not wrapped in a call.  This happens when moving a local

cmake/fmtlib.cmake

@@ -3,7 +3,7 @@ include(FetchContent)
 FetchContent_Declare(
 	fmtlib
 	PREFIX "${CMAKE_BINARY_DIR}/deps"
-	DOWNLOAD_DIR "${CMAKE_SOURCE_DIR}/deps/downloads"
+	DOWNLOAD_DIR "${PROJECT_SOURCE_DIR}/deps/downloads"
 	DOWNLOAD_NAME fmt-8.0.1.tar.gz
 	URL https://github.com/fmtlib/fmt/archive/8.0.1.tar.gz
 	URL_HASH SHA256=b06ca3130158c625848f3fb7418f235155a4d389b2abc3a6245fb01cb0eb1e01

cmake/jsoncpp.cmake

@@ -42,7 +42,7 @@ endif()
 
 ExternalProject_Add(jsoncpp-project
     PREFIX "${prefix}"
-    DOWNLOAD_DIR "${CMAKE_SOURCE_DIR}/deps/downloads"
+    DOWNLOAD_DIR "${PROJECT_SOURCE_DIR}/deps/downloads"
     DOWNLOAD_NAME jsoncpp-1.9.3.tar.gz
     URL https://github.com/open-source-parsers/jsoncpp/archive/1.9.3.tar.gz
     URL_HASH SHA256=8593c1d69e703563d94d8c12244e2e18893eeb9a8a9f8aa3d09a327aa45c8f7d

cmake/range-v3.cmake

@@ -11,7 +11,7 @@ set(RANGE_V3_INCLUDE_DIR "${prefix}/include")
 
 ExternalProject_Add(range-v3-project
     PREFIX "${prefix}"
-    DOWNLOAD_DIR "${CMAKE_SOURCE_DIR}/deps/downloads"
+    DOWNLOAD_DIR "${PROJECT_SOURCE_DIR}/deps/downloads"
     DOWNLOAD_NAME range-v3-0.12.0.tar.gz
     URL https://github.com/ericniebler/range-v3/archive/0.12.0.tar.gz
     URL_HASH SHA256=015adb2300a98edfceaf0725beec3337f542af4915cec4d0b89fa0886f4ba9cb