Clear XXX_FOUND in Find<XXX>.cmake prior to searching.

- This protects against a client project which invokes
  find_package(Ceres) after having called find_package(Foo) with their
  own version of FindFoo.cmake which conflicts with Ceres’ exported
  version and defines FOO_FOUND, but not the other variables Ceres’
  FindFoo.cmake is expecting which can break the detection logic.

Change-Id: I9fe7bfa8a34bb58b09ffe34446da973912cf5587

cmake/FindCXSparse.cmake

diff --git a/cmake/FindCXSparse.cmake b/cmake/FindCXSparse.cmake
index e6c21d2..ce01199 100644
--- a/cmake/FindCXSparse.cmake
+++ b/cmake/FindCXSparse.cmake
@@ -103,6 +103,12 @@
   return()
 endmacro(CXSPARSE_REPORT_NOT_FOUND)
 
+# Protect against any alternative find_package scripts for this library having
+# been called previously (in a client project) which set CXSPARSE_FOUND, but not
+# the other variables we require / set here which could cause the search logic
+# here to fail.
+unset(CXSPARSE_FOUND)
+
 # Handle possible presence of lib prefix for libraries on MSVC, see
 # also CXSPARSE_RESET_FIND_LIBRARY_PREFIX().
 if (MSVC)

cmake/FindEigen.cmake

diff --git a/cmake/FindEigen.cmake b/cmake/FindEigen.cmake
index 5d8ecda..d45fad7 100644
--- a/cmake/FindEigen.cmake
+++ b/cmake/FindEigen.cmake
@@ -83,6 +83,12 @@
   return()
 endmacro(EIGEN_REPORT_NOT_FOUND)
 
+# Protect against any alternative find_package scripts for this library having
+# been called previously (in a client project) which set EIGEN_FOUND, but not
+# the other variables we require / set here which could cause the search logic
+# here to fail.
+unset(EIGEN_FOUND)
+
 # Search user-installed locations first, so that we prefer user installs
 # to system installs where both exist.
 #

cmake/FindGflags.cmake

diff --git a/cmake/FindGflags.cmake b/cmake/FindGflags.cmake
index 751771d..0bd5c56 100644
--- a/cmake/FindGflags.cmake
+++ b/cmake/FindGflags.cmake
@@ -271,6 +271,12 @@
   endif()
 endfunction(GFLAGS_CHECK_GFLAGS_NAMESPACE_USING_REGEX)
 
+# Protect against any alternative find_package scripts for this library having
+# been called previously (in a client project) which set GFLAGS_FOUND, but not
+# the other variables we require / set here which could cause the search logic
+# here to fail.
+unset(GFLAGS_FOUND)
+
 # -----------------------------------------------------------------
 # By default, if the user has expressed no preference for using an exported
 # gflags CMake configuration over performing a search for the installed

cmake/FindGlog.cmake

diff --git a/cmake/FindGlog.cmake b/cmake/FindGlog.cmake
index 17b8bc5..3411685 100644
--- a/cmake/FindGlog.cmake
+++ b/cmake/FindGlog.cmake
@@ -109,6 +109,12 @@
   return()
 endmacro(GLOG_REPORT_NOT_FOUND)
 
+# Protect against any alternative find_package scripts for this library having
+# been called previously (in a client project) which set GLOG_FOUND, but not
+# the other variables we require / set here which could cause the search logic
+# here to fail.
+unset(GLOG_FOUND)
+
 # -----------------------------------------------------------------
 # By default, if the user has expressed no preference for using an exported
 # glog CMake configuration over performing a search for the installed

cmake/FindSuiteSparse.cmake

diff --git a/cmake/FindSuiteSparse.cmake b/cmake/FindSuiteSparse.cmake
index 6815c03..af1fb1d 100644
--- a/cmake/FindSuiteSparse.cmake
+++ b/cmake/FindSuiteSparse.cmake
@@ -152,6 +152,12 @@
   # the first.
 endmacro(SUITESPARSE_REPORT_NOT_FOUND)
 
+# Protect against any alternative find_package scripts for this library having
+# been called previously (in a client project) which set SUITESPARSE_FOUND, but
+# not the other variables we require / set here which could cause the search
+# logic here to fail.
+unset(SUITESPARSE_FOUND)
+
 # Handle possible presence of lib prefix for libraries on MSVC, see
 # also SUITESPARSE_RESET_FIND_LIBRARY_PREFIX().
 if (MSVC)