ENH: Allow fine grained control over dependancies

An end user may want to explicitly avoid using
a feature that can be automatically found on the
build system.  This arose when the build machine
had libraries for LZMA but the target machine
did not have those libraries available.

By allowing flags the optional features provided by
LZMA/ZLIB/BZip2/EXPAT/PCREPOSIX/LibGCC
can be explicitly removed (Default behavior is
to use what can be found).

CMakeLists.txt

@@ -143,6 +143,13 @@ include(CTest)
 
 OPTION(ENABLE_NETTLE "Enable use of Nettle" ON)
 OPTION(ENABLE_OPENSSL "Enable use of OpenSSL" ON)
+OPTION(ENABLE_LZMA "Enable the use of the system found LZMA library if found" ON)
+OPTION(ENABLE_ZLIB "Enable the use of the system found ZLIB library if found" ON)
+OPTION(ENABLE_BZip2 "Enable the use of the system found BZip2 library if found" ON)
+OPTION(ENABLE_EXPAT "Enable the use of the system found EXPAT library if found" ON)
+OPTION(ENABLE_PCREPOSIX "Enable the use of the system found PCREPOSIX library if found" ON)
+OPTION(ENABLE_LibGCC "Enable the use of the system found LibGCC library if found" ON)
+
 OPTION(ENABLE_TAR "Enable tar building" ON)
 OPTION(ENABLE_TAR_SHARED "Enable dynamic build of tar" FALSE)
 OPTION(ENABLE_CPIO "Enable cpio building" ON)
@@ -151,6 +158,7 @@ OPTION(ENABLE_XATTR "Enable extended attribute support" ON)
 OPTION(ENABLE_ACL "Enable ACL support" ON)
 OPTION(ENABLE_ICONV "Enable iconv support" ON)
 OPTION(ENABLE_TEST "Enable unit and regression tests" ON)
+
 SET(POSIX_REGEX_LIB "AUTO" CACHE STRING "Choose what library should provide POSIX regular expression support")
 SET(ENABLE_SAFESEH "AUTO" CACHE STRING "Enable use of /SAFESEH linker flag (MSVC only)")
 SET(WINDOWS_VERSION "" CACHE STRING "Set Windows version to use (Windows only)")
@@ -315,7 +323,11 @@ SET(ADDITIONAL_LIBS "")
 #
 # Find ZLIB
 #
-FIND_PACKAGE(ZLIB)
+IF(ENABLE_ZLIB)
+  FIND_PACKAGE(ZLIB)
+ELSE()
+  SET(ZLIB_FOUND FALSE) # Override cached value
+ENDIF()
 IF(ZLIB_FOUND)
   SET(HAVE_LIBZ 1)
   SET(HAVE_ZLIB_H 1)
@@ -350,7 +362,11 @@ MARK_AS_ADVANCED(CLEAR ZLIB_LIBRARY)
 #
 # Find BZip2
 #
-FIND_PACKAGE(BZip2)
+IF(ENABLE_BZip2)
+  FIND_PACKAGE(BZip2)
+ELSE()
+  SET(BZIP2_FOUND FALSE) # Override cached value
+ENDIF()
 IF(BZIP2_FOUND)
   SET(HAVE_LIBBZ2 1)
   SET(HAVE_BZLIB_H 1)
@@ -370,10 +386,18 @@ IF(BZIP2_FOUND)
 ENDIF(BZIP2_FOUND)
 MARK_AS_ADVANCED(CLEAR BZIP2_INCLUDE_DIR)
 MARK_AS_ADVANCED(CLEAR BZIP2_LIBRARIES)
+
+
 #
 # Find LZMA
 #
-FIND_PACKAGE(LZMA)
+IF(ENABLE_LZMA)
+  FIND_PACKAGE(LZMA)
+ELSE()
+  SET(LZMA_FOUND FALSE) # Override cached value
+  SET(LZMADEC_FOUND FALSE) # Override cached value
+ENDIF()
+
 IF(LZMA_FOUND)
   SET(HAVE_LIBLZMA 1)
   SET(HAVE_LZMA_H 1)
@@ -393,6 +417,8 @@ ELSEIF(LZMADEC_FOUND)
   SET(HAVE_LZMADEC_H 1)
   INCLUDE_DIRECTORIES(${LZMADEC_INCLUDE_DIR})
   LIST(APPEND ADDITIONAL_LIBS ${LZMADEC_LIBRARIES})
+ELSE(LZMA_FOUND)
+# LZMA not found and will not be used.
 ENDIF(LZMA_FOUND)
 #
 # Find LZO2
@@ -871,8 +897,8 @@ ENDIF(ENABLE_ICONV)
 
 #
 # Find Libxml2
+FIND_PACKAGE(LibXml2 REQUIRED)
 #
-FIND_PACKAGE(LibXml2)
 IF(LIBXML2_FOUND)
   CMAKE_PUSH_CHECK_STATE()	# Save the state of the variables
   INCLUDE_DIRECTORIES(${LIBXML2_INCLUDE_DIR})
@@ -967,8 +993,16 @@ IF(NOT FOUND_POSIX_REGEX_LIB AND POSIX_REGEX_LIB MATCHES "^(AUTO|LIBPCREPOSIX)$"
   #
   # If requested, try finding library for PCREPOSIX
   #
-  FIND_PACKAGE(LibGCC)
-  FIND_PACKAGE(PCREPOSIX)
+  IF(ENABLE_LibGCC)
+    FIND_PACKAGE(LibGCC)
+  ELSE()
+    SET(LIBGCC_FOUND FALSE) # Override cached value
+  ENDIF()
+  IF(ENABLE_PCREPOSIX)
+    FIND_PACKAGE(PCREPOSIX)
+  ELSE()
+    SET(PCREPOSIX_FOUND FALSE) # Override cached value
+  ENDIF()
   IF(PCREPOSIX_FOUND)
     INCLUDE_DIRECTORIES(${PCRE_INCLUDE_DIR})
     LIST(APPEND ADDITIONAL_LIBS ${PCREPOSIX_LIBRARIES})