cmake: fix up source and binary directory paths

There are quite some uses of the variables "${CMAKE_CURRENT_SOURCE_DIR}"
and "${CMAKE_CURRENT_BINARY_DIR}" where they are not appropriate.
Convert these sites to instead use the variables "${CMAKE_SOURCE_DIR}"
and "${CMAKE_BINARY_DIR}", which instead point to the project's root
directory.

This will ease splitting up the library build instructions into its own
subdirectory.

CMakeLists.txt

@@ -19,7 +19,7 @@ IF (CMAKE_VERSION VERSION_GREATER 3.0)
 ENDIF()
 
 # Add find modules to the path
-SET(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules/")
+SET(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")
 
 INCLUDE(CheckLibraryExists)
 INCLUDE(CheckFunctionExists)
@@ -161,7 +161,7 @@ FUNCTION(IDE_SPLIT_SOURCES target)
 		GET_TARGET_PROPERTY(sources ${target} SOURCES)
 		FOREACH(source ${sources})
 			IF(source MATCHES ".*/")
-				STRING(REPLACE ${CMAKE_CURRENT_SOURCE_DIR}/ "" rel ${source})
+				STRING(REPLACE ${CMAKE_SOURCE_DIR}/ "" rel ${source})
 				IF(rel)
 					STRING(REGEX REPLACE "/([^/]*)$" "" rel ${rel})
 					IF(rel)
@@ -174,14 +174,14 @@ FUNCTION(IDE_SPLIT_SOURCES target)
 	ENDIF()
 ENDFUNCTION()
 
-FILE(STRINGS "include/git2/version.h" GIT2_HEADER REGEX "^#define LIBGIT2_VERSION \"[^\"]*\"$")
+FILE(STRINGS "${CMAKE_SOURCE_DIR}/include/git2/version.h" GIT2_HEADER REGEX "^#define LIBGIT2_VERSION \"[^\"]*\"$")
 
 STRING(REGEX REPLACE "^.*LIBGIT2_VERSION \"([0-9]+).*$" "\\1" LIBGIT2_VERSION_MAJOR "${GIT2_HEADER}")
 STRING(REGEX REPLACE "^.*LIBGIT2_VERSION \"[0-9]+\\.([0-9]+).*$" "\\1" LIBGIT2_VERSION_MINOR  "${GIT2_HEADER}")
 STRING(REGEX REPLACE "^.*LIBGIT2_VERSION \"[0-9]+\\.[0-9]+\\.([0-9]+).*$" "\\1" LIBGIT2_VERSION_REV "${GIT2_HEADER}")
 SET(LIBGIT2_VERSION_STRING "${LIBGIT2_VERSION_MAJOR}.${LIBGIT2_VERSION_MINOR}.${LIBGIT2_VERSION_REV}")
 
-FILE(STRINGS "include/git2/version.h" GIT2_HEADER_SOVERSION REGEX "^#define LIBGIT2_SOVERSION [0-9]+$")
+FILE(STRINGS "${CMAKE_SOURCE_DIR}/include/git2/version.h" GIT2_HEADER_SOVERSION REGEX "^#define LIBGIT2_SOVERSION [0-9]+$")
 STRING(REGEX REPLACE "^.*LIBGIT2_SOVERSION ([0-9]+)$" "\\1" LIBGIT2_SOVERSION "${GIT2_HEADER_SOVERSION}")
 
 # Enable tracing
@@ -567,7 +567,10 @@ ENDIF()
 ADD_DEFINITIONS(-D_FILE_OFFSET_BITS=64)
 
 # Collect sourcefiles
-FILE(GLOB SRC_H include/git2.h include/git2/*.h include/git2/sys/*.h)
+FILE(GLOB SRC_H
+	"${CMAKE_SOURCE_DIR}/include/git2.h"
+	"${CMAKE_SOURCE_DIR}/include/git2/*.h"
+	"${CMAKE_SOURCE_DIR}/include/git2/sys/*.h")
 
 # On Windows use specific platform sources
 IF (WIN32 AND NOT CYGWIN)
@@ -645,7 +648,7 @@ IF (SONAME)
 	ENDIF()
 ENDIF()
 STRING(REPLACE ";" " " LIBGIT2_PC_LIBS "${LIBGIT2_PC_LIBS}")
-CONFIGURE_FILE(${CMAKE_CURRENT_SOURCE_DIR}/libgit2.pc.in ${CMAKE_CURRENT_BINARY_DIR}/libgit2.pc @ONLY)
+CONFIGURE_FILE(${CMAKE_SOURCE_DIR}/libgit2.pc.in ${CMAKE_BINARY_DIR}/libgit2.pc @ONLY)
 
 IF (MSVC_IDE)
    # Precompiled headers
@@ -659,9 +662,9 @@ INSTALL(TARGETS git2
 	LIBRARY DESTINATION ${LIB_INSTALL_DIR}
 	ARCHIVE DESTINATION ${LIB_INSTALL_DIR}
 )
-INSTALL(FILES ${CMAKE_CURRENT_BINARY_DIR}/libgit2.pc DESTINATION ${LIB_INSTALL_DIR}/pkgconfig )
-INSTALL(DIRECTORY include/git2 DESTINATION ${INCLUDE_INSTALL_DIR} )
-INSTALL(FILES include/git2.h DESTINATION ${INCLUDE_INSTALL_DIR} )
+INSTALL(FILES ${CMAKE_BINARY_DIR}/libgit2.pc DESTINATION ${LIB_INSTALL_DIR}/pkgconfig )
+INSTALL(DIRECTORY ${CMAKE_SOURCE_DIR}/include/git2 DESTINATION ${INCLUDE_INSTALL_DIR} )
+INSTALL(FILES ${CMAKE_SOURCE_DIR}/include/git2.h DESTINATION ${INCLUDE_INSTALL_DIR} )
 
 # Tests
 IF (BUILD_CLAR)