libfolly: Add package for folly library

folly is an open-source C++ library developed and used at Facebook.
Adding this package will enable adding further packages that depend on
this.

Depends on PR #7098, PR #7101, PR #7126, PR #7877

Maintainer: me
Compile tested: openwrt-18.06 for ipq40xx, ipq806x, x86 and ar71xx
Run tested: Tested on devices running on the above architectures.
Verified that dependent packages can successfully build and run using
the libfolly.so shared library.

Signed-off-by: Amol Bhave <ambhave@fb.com>

libs/libfolly/patches/100-cmake-fix-regex-match-path.patch

@@ -0,0 +1,40 @@
+Index: folly-2019.05.06.00/CMakeLists.txt
+===================================================================
+--- folly-2019.05.06.00.orig/CMakeLists.txt
++++ folly-2019.05.06.00/CMakeLists.txt
+@@ -109,17 +109,19 @@ configure_file(
+ auto_sources(files "*.cpp" "RECURSE" "${FOLLY_DIR}")
+ auto_sources(hfiles "*.h" "RECURSE" "${FOLLY_DIR}")
+
++string(REPLACE + \\+ FOLLY_DIR_ESCAPED ${FOLLY_DIR})
++
+ # Exclude tests, benchmarks, and other standalone utility executables from the
+ # library sources.  Test sources are listed separately below.
+ REMOVE_MATCHES_FROM_LISTS(files hfiles
+   MATCHES
+-    "^${FOLLY_DIR}/build/"
+-    "^${FOLLY_DIR}/experimental/exception_tracer/"
+-    "^${FOLLY_DIR}/experimental/pushmi/"
+-    "^${FOLLY_DIR}/futures/exercises/"
+-    "^${FOLLY_DIR}/logging/example/"
+-    "^${FOLLY_DIR}/(.*/)?test/"
+-    "^${FOLLY_DIR}/tools/"
++    "^${FOLLY_DIR_ESCAPED}/build/"
++    "^${FOLLY_DIR_ESCAPED}/experimental/exception_tracer/"
++    "^${FOLLY_DIR_ESCAPED}/experimental/pushmi/"
++    "^${FOLLY_DIR_ESCAPED}/futures/exercises/"
++    "^${FOLLY_DIR_ESCAPED}/logging/example/"
++    "^${FOLLY_DIR_ESCAPED}/(.*/)?test/"
++    "^${FOLLY_DIR_ESCAPED}/tools/"
+     "Benchmark.cpp$"
+     "Test.cpp$"
+ )
+@@ -169,7 +171,7 @@ list(APPEND hfiles
+ if (NOT FOLLY_USE_SYMBOLIZER)
+   REMOVE_MATCHES_FROM_LISTS(files hfiles
+     MATCHES
+-      "^${FOLLY_DIR}/experimental/symbolizer/"
++      "^${FOLLY_DIR_ESCAPED}/experimental/symbolizer/"
+   )
+   list(REMOVE_ITEM files
+     ${FOLLY_DIR}/SingletonStackTrace.cpp