Do not use relative paths in build system

Summary: Previously, RocksDB's build scripts used relative pathnames like ./build_detect_platform. This can cause problems if the user uses CDPATH. Also, it just doesn't seem right to me.

Test Plan:
make clean
make -j32 check

Reviewers: MarkCallaghan, dhruba, kailiu

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12459

Makefile

@@ -14,7 +14,7 @@ OPT += -O2 -fno-omit-frame-pointer -momit-leaf-frame-pointer
 #-----------------------------------------------
 
 # detect what platform we're building on
-$(shell (cd build_tools/; ROCKSDB_ROOT=.. ./build_detect_platform ../build_config.mk))
+$(shell (export ROCKSDB_ROOT=$(CURDIR); $(CURDIR)/build_tools/build_detect_platform $(CURDIR)/build_config.mk))
 # this file is generated by the previous line to set build flags and sources
 include build_config.mk
 

build_tools/build_detect_platform

@@ -31,9 +31,9 @@ fi
 # Default to fbcode gcc on internal fb machines
 if [ -d /mnt/gvfs/third-party -a -z "$CXX" ]; then
     if [ -z "$USE_CLANG" ]; then
-        source ./fbcode.gcc471.sh
+        source $PWD/build_tools/fbcode.gcc471.sh
     else
-        source ./fbcode.clang31.sh
+        source $PWD/build_tools/fbcode.clang31.sh
     fi
 fi
 
@@ -65,7 +65,7 @@ PLATFORM_SHARED_CFLAGS="-fPIC"
 PLATFORM_SHARED_VERSIONED=true
 
 # generic port files (working on all platform by #ifdef) go directly in /port
-GENERIC_PORT_FILES=`cd $ROCKSDB_ROOT; find port -name '*.cc' | tr "\n" " "`
+GENERIC_PORT_FILES=`find $ROCKSDB_ROOT/port -name '*.cc' | tr "\n" " "`
 
 # On GCC, we pick libc's memcmp over GCC's memcmp via -fno-builtin-memcmp
 case "$TARGET_OS" in
@@ -127,7 +127,7 @@ case "$TARGET_OS" in
         exit 1
 esac
 
-./build_detect_version
+$PWD/build_tools/build_detect_version
 
 # We want to make a list of all cc files within util, db, table, and helpers
 # except for the test and benchmark files. By default, find will output a list